Adware
Adware, like Malware and Spyware, is a very general term that can refer to a range of programs. The common feature in all of them is that they expose you to advertising. Believe it or not, not all Adware is bad. Whenever you use an application or visit a site that has an advertising bar somewhere on the page, that's technically Adware. Like paid-for broadcast time on TV, these bars and windows are paid for by vendors that help offset the cost to the webpage's host. Occasionally people download free software that has advertising bars or windows as well, and though these flashing ads may be annoying, most users are willing to deal with them for sake of using the application without cost.

But nowadays, when someone says Adware, they're probably talking about a specific group of malicious program. This Adware comes in two forms. The first is not so insidious but skirts vendor/user ethics. These are ad-displays bundled to software that doesn't announce their presence clearly or up-front. The clause that says you're downloading advertising may be mentioned in the download wizard, but maybe it won't be. Or maybe the ad notice will be in tiny print in the End User License Agreement (The long I Accept form that few read in full) . . . or maybe it won't be.

And then there's some Adware that's just plain bad. This is malicious software that comes bundled to legitimate programs or is embedded into certain websites. This Adware can hijack your browser or homepage, veering you to some pre-specified site, or it can bombard you with the most classic and detested form of Adware, the popup. What's more, since this type of Adware is downloaded secretly, it's often being carried in the payload of a Trojan, and while the Adware itself is more annoying than harmful, it probably did not come alone.

Backdoor
Backdoors are a form of Trojan that is formidably dangerous. In the simplest terms, Backdoors are programs that open up a hole, or backdoor, into a user's computer and allow another party (called a Master) to control the computer remotely through a RAT (Remote Administrator Tool). RATs are useful functions that allow legitimate operators, such as an installer or tech supporter, to literally control your computer from a distance, possible even overseas. But in the hands of a malicious hacker, this function becomes a portal, allowing the master to:

• view confidential information
• execute malicious codes
• launch programs
• send / receive date via the Internet
• delete files and other data
• display notifications
• reboot the system

Browser Helper Object
Browser Helper Objects or BHOs are another kind of program that, on the whole, offers a positive service. They are designed to augment certain browsers (usually Internet Explorer), allowing them to do more specialized tasks. For example, the Google Toolbar in Explorer is a BHO that allows IE users to search Google no matter what page they're on.

Yet BHOs are also susceptible to abuse. Certain BHOs offer legitimate features (such as search engine toolbars) but behind the scenes record where it is that you go and relay that information to advertisers or scammers. BHOs can even be potentially dangerous, especially if they work in tandem with another form of Malware like a Trojan. There have been cases wherein certain BHOs recognized whenever a user accessed a secure site. The BHO then signaled a keylogger to start recording the keystrokes so that a 3rd party could see any potential passwords.

Commercial RAT
The RAT stands for Remote Administration Tool, and, on the whole, commercial RATs are very useful utilities. As the name suggests, these programs allow remote access to your computer. If you need an installation done or have a tech support question, many helpful associates can utilize a RAT to take over your computer and help with the problem. This can be done from across the room or across an ocean. Needless to say a RAT in the hands of the wrong person could be devastating as he or she could control all of your computer's functions and data. From shutting down programs to deleting files to deleting the hard drive, an unauthorized hacker can wreak havoc on an unsuspecting system.

When hackers gain control through a RAT, they are called Masters, and the easiest way they can gain that control is through a Backdoor Trojan program.

Data Miner
A Data Miner is a general categorization for any utility that simply collects data and then transmits it from your computer to an unseen 3rd party. Data miners can log a multitude of simple things, such as email addresses or web searches, into a respectively small space, including basic text files. Hence, Tracking Cookies can be considered Data Miners.

Dialer
A Dialer a piece of software or hardware that literally dials a modem onto an analog phone line. Anyone who uses a dial-up connection to the Internet has a dialer program in their computer. When people refer to a dialer as a kind of Malware though, they're referencing a type of fraud propagated by unscrupulous programmers. These programmers write Dialers that log a computer on to special content numbers, equivocal to the premium 1-900 numbers one can dial on a telephone. Needless to say, these numbers come at a high cost, often with charges that are either obscured or hidden from the user.

Many of these dubious dialer programs are remarkably up-front. You can get them from websites that tell you exactly what they are: extra cost numbers that give the user access to special content. This can be anything from pornography to (often illegal) music downloads to hacking material. But whereas these dialers tell you that you're paying extra, they often include hidden fees and minimum time requirements that gouge the unsuspecting.

Some Dialers are simply malevolent and can be dropped by a Trojan unknowingly into a user's system. These can actually hijack a computer's number bank and plug in a series of these premium numbers. The user will log on not knowing that anything has changed, especially when some of these premium numbers go to the normal Internet. The result is an unsuspecting victim who has no idea what's happened . . . until the bill comes.

Downloader
As its name implies a Downloader is the part of a malicious program that actually downloads and installs software onto your computer. Downloaders can be found on any type of malware, but they're most commonly associated with Trojans because these programs' objective is to download material without the user being aware of it.

The Downloader is usually responsible for executing the programs it has loaded. It may do this as soon as the downloading process is complete, or it may register the execute command with the local system requirements. This way the the malicious software will autorun at a specified time or after a specified action. The names of the downloaded files and the locations may be either encoded in the Downloader itself or sent via an unseen website.

Email-Worm
Email Worms, like all worms, an email worm is a self-propagating (self replicating) program. This one, logically, is contracted via email. These are perhaps the most recognizable form of malware since almost anyone with an email address has come across their fair share of bogus emails, many of which contain harmful programs.

Not too long ago people began to be very concerned about opening email attachments. These worms were the reason. Like any malicious software, this worm is a program and can only function if it is executed, so it took an active gesture from a user, such as opening the attachment, to install itself on a computer. There are, however, some more complicated email worms that can install themselves by being laced into the html rendering of an email's body. Thus, just reading the email puts the user at some risk, especially if the email client (programs like Microsoft Outlook) had a viewing pane that showed the email if it was simply highlighted in the inbox.

Fortunately, most modern email clients offer plain text rendering of their emails, so it cannot harbor malicious code. Worm makers are wily though, and recently email worms have made a resurgence thanks to a process dubbed social engineering, a method that employs more trickery than coding muscle to get a user to install the worm. Scams such as phishing are particularly successful in getting unsuspecting users to divulge vital information and open themselves up to these worms.

For the most part, email today is much safer than it used to be, but good rules of caution are still needed. Don't open attachments from people/organizations you do not know and be wary of official looking emails that ask for vital information (such as account numbers or credit card numbers). No reputable organization ever asks for these via email, only on their secure sites. And remember, when in doubt, pick up the phone and ask.

Firewall Killer
The general philosophy behind Spyware is defensive: if it stays undetected, it won't be removed. But occasionally there comes along some software that takes an offense stance, attacking computer Firewall security. Some varieties can even attack anti-Spyware scanners, making them forget or ignore the Spyware when they find it.

The most insidious versions of Firewall Killers can even throw up dummy Firewalls to make the computer think that nothing is wrong. The only way to combat relatively advanced software such as this is to install advanced anti-Spyware software.

Flooder
When a website host or business server receives a download request, sometimes the connection to the requesting machine is incomplete. When this happens, this request goes into the server's memory buffer, a protective storage space for useless requests that can hold them while legitimate connections are made in the mean time. But this memory buffer is only so big. If hundreds and hundreds of computers all sent incomplete connection requests all at the same time, the buffer would fill up and the server would stop functioning all together resulting in a Denial of Service. This is what's known as a Denial of Service or DoS attack.

These attacks are orchestrated by a kind of Trojan program called a Flooder that literally floods the server with these junk connection requests. Where do the Flooders live? They may be on your computer. A Trojan-Proxy is a kind of Spyware that allows an unseen hacker to use your computer as a launching point for various kids of data. These include spam and DoS attack packets. When enough individual computers become infected, the hacker gives the signal and the installed Flooders go to work, making your computer now part of an illegal attack on someone else's system.

Hijacker
Hijackers are specialized programs that take over or hijack your Browser Homepage. Hijackers are one of the more recognizable and detested forms of Adware because the sites that become your Homepage are often annoying and sometimes profane. Trying to reset the Homepage while the Hijacker is still in the system is nigh to impossible. Moreover, since Hijackers are usually one of several programs dropped by a Trojan, finding one in your browser usually means there are other Spyware programs working behind the scenes.

Key Logger
A keylogger or keystroke logger is a program that literally logs what keys are being typed. The potential damage this can do is fairly obvious, a snooper who has access to the keylog can potentially see account numbers, passwords, or any given text that may contain sensitive material. Often keyloggers are simple programs so they can be dropped with other types of Spyware in the payload of a Trojan.

There is actually a hardware version of a keylogger that usually manifests as a sabotaged keyboard with a recording device built into it. But since a Spyware maker would have to have physical access to your computer, these are rare not a real threat.

Malware
Malware is a general categorization more than a specific kind of program. It denotes any software that damages, spies on, controls, or otherwise manipulates a user's computer data without the user's consent. The term comes from the Latin malus, mala which translates as bad, wicked, or evil.

Hence Viruses, Worms, Trojans, Backdoors, Rootkits, Logic Bombs, etc. all fit under this category. In addition, programs that are not malicious in themselves but aid in the creation of malicious software are also considered malware since they indirectly accomplish the same thing.

p2p
P2P stands for Peer to Peer. These are file-sharing networks that allow users to literally access one another's computers without having a server in between. While these networks provide amazing possibilities for data sharing, it's not hard to see why they are also a fertile breeding ground for malicious applications. Sometimes users purposely bundle Spyware or Adware onto files shared P2P. And sometimes users simply spread these programs unwittingly, sharing and using and sharing files bundled with self-propagating worms.

Potentially Unwanted Applications
Spyware rarely comes as a single program. More often than not, if a Spyware programmer can bundle several malicious programs together, the more opportunity he or she has to snoop, steal or hijack.

Sometimes though, there are programs that don't seem to do anything wrong or damaging. They aren't Spyware per se, nor are they legitimate hoax programs meant to cover up the Spyware. They're just suspicious because of the crowd they travel with. For this reason, programs like this are often referred to as potentially unwanted applications or PUAs.

Rogue AntiSpyware
The sad truth is there are a multitude of Spyware products that masquerade as legitimate anti-Spyware scanners. They look like the real thing, they scan like the real thing, but they often don't truly remove any significant threats. They eat up memory space, and they are difficult if impossible to uninstall. What's more, the anti-Spyware scanner often acts as the smokescreening hoax program that distracts you from genuinely malicious applications, running in the background.

There is a second type of Rogue anti-Spyware and these are Rogues that do not have malicious intent but simply do not function correctly. An anti-Spyware scanner is analogous to a surgeon, cutting out infected files in very sensitive areas of your computer. If a scanner is clumsy or carelessly designed, it may cut out valuable (or vital) parts.

Rootkit
Rootkits are, in essence, a smokescreen designed to hide the activities of a hacker. The name comes from an older technique wherein a hacker would attack a Unix server with the hope of achieving administrative (or Root) privileges. The hacker would begin by seeking a vulnerability in a low-level user account and throwing up this so-called diversion while he or she collected user ids until the root access was gained. The kit part of the system was a series of programs that made it seem as if nothing was wrong. By installing a kernel module or replacing system files or system libraries, the hacker prevented a user from realizing the security had been compromised.

Today, the Rootkit method is used on Windows systems as well, and since most Windows users have administrative accounts, the process is all the simpler. Though it is no longer called root access, the process is the same so the name has stuck.

Spyware
Spyware is a term that encompasses a broad range of undesirable programs that may infect your computer. If a program invades your privacy by allowing someone else to eavesdrop on your computer activity, it falls into the Spyware category. Some examples of Spyware include keyloggers and tracking cookies. Almost every single computer connected to the Web has been or will be attacked by Spyware of some form. The only way to protect yourself and your privacy is to run a comprehensive anti-spyware program on your system at all times.

Tracking Cookie
Cookies are (usually) small data packets that store simple information about a user and communicate that information to a web server. In essence, this is a good thing. It means that certain websites can have their servers recognize your computer when you visit them. If you've ever had a website remember your login name, that's a cookie. If you've ever shopped using a shopping cart online, the cookie allows you to keep the shopping cart unchanged in the background while you continue to shop.

Since a cookie isn't a program, it cannot execute any harmful commands, and many people needlessly delete every Cookie they find. Deleting a Cookie doesn't hurt anything, but it does mean that when you revisit certain sites you won't be recognized. So what's the big deal then? Although most Cookies are harmless (or even helpful) some are designed to record your web activity, such as where you go online and how frequently. These Cookies are then accessed by snooping 3rd parties who can learn about you, where you go and when.

These so-called Tracking Cookies are generally considered malevolent since they can facilitate someone finding out sensitive information, such as where you bank or what sorts of stores you patron. Once these 3rd parties know this information, they can target you for advertising or scams.

Trojan
A Trojan is a shortened handle for Trojan Horse. The name, as you've probably guessed, hails from Homer's Iliad wherein Odysseus concocts a plan to defeat the Trojans by building a giant horse and stashing armed warriors inside. Once presented to the king of Troy as a gesture of surrender, the horse is brought into the city. The analogy to this type of Malware is highly apt. Trojans are malicious programs that are always attached to something legitimate. The user unwittingly downloads the game, or music, or even the rogue anti-Spyware utility, and then the Trojan that was unseen will go to work.

Any malicious program that doesn't want to be heard or seen is usually classified as a type of Trojan. That being said, there are lots of types of Trojans that have various goals. Some Trojans install Spyware; some turn your computer into a Proxy, a launchpad for spam; some install Adware; and some even open a Backdoor on your computer. This is an access point for someone to come in and literally take control of all your computer's functions and data. Although Trojans do not replicate themselves like worms, the payloads they unleash can be just as damaging.

Trojan-Clicker
A Trojan clicker is a malicious program that is bundled with a seemingly legitimately program. The mission of the clicker is to redirect you to some website that you don't specify. It can click you to this site in one of two ways. The first involves the Trojan-Clicker actually sending commands directly to the browser, telling it where to go. The other method involves the Clicker replacing system files where URLs are stored.

A Trojan-Clicker can have invasive but benign intentions, such as raising a website's hit counter for advertising purposes. Or it can have the more malicious goal of leading the victim machine to a site that can infect it with more malware.

Trojan-Dropper
Trojan-Droppers are one of the most common types of Trojans and is itself a pretty general category. The Droppers are malicious programs that drop various smaller Trojans into different parts of the computer, usually with the intent to spy on user activities. In addition to its subversive payload, the Dropper frequently carries at least one hoax program. The hoax is a functioning application, such as a joke generator, music file, video file, graphic, or even game. This application will make it seem as if the download has been legitimate, and it can also thwart some anti-Spyware applications, making them think that the hoax is all there is.

A frequent sign of a Dropper is a faulty error message that looks official but isn't. This message is a distraction that makes a user think that any strange behavior is a glitch the software or the OS.

Trojan-Proxy
Trojan-Proxies are best understood as relays. When unscrupulous advertisers want to send out lots of spam, they need as many addresses and as much bandwidth as they can get. The Proxies allow these spammers to use your computer as a launching point for more spam, often using your own email contacts as targets. Hence, this Trojan makes your computer not just a victim of malicious activity but an active participant since the proxy is sending the spam out through your own email account.

Not only is this attack an invasion of privacy and a nuisance to your email contacts, it also has the potential to involve your computer in malicious, or even illegal, activity.

Virus
Virus is a general term for a code that executes any malicious intent in a victim computer. Usually a virus's objective is to either open vital resources (and thus expose crucial data to theft or attack), or to execute a specified command once the user has fulfilled a specific sequence of actions. Unlike a Worm, a Virus does not propagate itself over a LAN or the Internet. It has to infect a host program or file or disk that then comes into contact with the victim machine. Since they are programs, Viruses can only be activated when a user (either knowingly or unknowingly) accesses the infected medium and executes the malicious code. Hence a virus is usually spread by

• Being launched from an infected file on a network resource accessed by other users.
• Being launched from an infected email attachment.
• Being launched from an infected storage media (such as a floppy disk, cd, or flash drive).

Worm
Worms are classified by their propensity to duplicate themselves. Trojans and Viruses don't do this, but a worm is adept at making so many versions of itself that it's hard to remove. Due to their prolific nature, worms can propagate themselves over the Internet and over a LAN without a specific command from the malicious programmer. Moreover, worms do not have to infect a file as a Virus does, so, like a Trojan, a worm can be downloaded with a legitimate program that can still function properly and independently. Email attachments, instant message attachments, FTP file shares and p2p file shares are all easy carriers for worms, which is why a user must exercise caution when dealing with any of these.

Like their non-propagating cousins, Trojans, worms usually have a variety of tasks that include opening up vital data to theft, launching popups and other advertising (adware), hijacking browsers, and spying on user activities (spyware).

Supplementary Resources

Anti Virus Rants
Kasperky's Viruslist.com - General Malware Types
Kasperky's Viruslist.com - Specific Malware Types