It seems that no sooner do you feel safe turning on your computer than you hear on the news about a new kind of internet security threat. Usually, the security threat is some kind of malware (though the term "security threat" no doubt sells more newspapers).

What is malware? Malware is exactly what its name implies: mal (meaning bad, in the sense of malignant or malicious rather than just poorly done) ware (short for software). More specifically, malware is software that does not benefit the computer's owner, and may even harm it, and so is purely parasitic.

The Many Faces of Malware

According to Wikipedia, there are in fact eleven distinct types of malware, and even more sub-types of each.

1. Viruses. The malware that's on the news so much, even your grandmother knows what it is. You probably already have heard plenty about why this kind of software is bad for you, so there's no need to belabor the point.

2. Worms. Slight variation on viruses. The difference between viruses and worms is that viruses hide inside the files of real computer programs (for instance, the macros in Word or the VBScript in many other Microsoft applications), while worms do not infect a file or program, but rather stand on their own.

3. Wabbits.Be honest: had you ever even heard of wabbits before (outside of Warner Bros. cartoons)? According to Wikipedia, wabbits are in fact rare, and it's not hard to see why: they don't do anything to spread to other machines. A wabbit, like a virus, replicates itself, but it does not have any instructions to email itself or pass itself through a computer network in order to infect other machines. The least ambitious of all malware, it is content simply to focus on utterly devastating a single machine.

4. Trojans. Arguably the most dangerous kind of malware, at least from a social standpoint. While Trojans rarely destroy computers or even files, that's only because they have bigger targets: your financial information, your computer's system resources, and sometimes even massive denial-of-service attacks launched by having thousands of computers all try to connect to a web server at the same time. Trojans can even

5. Spyware. In another instance of creative software naming, spyware is software that spies on you, often tracking your internet activities in order to serve you advertising. (Yes, it's possible to be both adware and spyware at the same time.)

6. Backdoors. Backdoors are much the same as Trojans or worms, except that they do something different: they open a "backdoor" onto a computer, providing a network connection for hackers or other malware to enter or for viruses or spam to be sent out through.

7. Exploits. Exploits attack specific security vulnerabilities. You know how Microsoft is always announcing new updates for its operating system? Often enough the updates are really trying to close the security hole targeted in a newly discovered exploit.

8. Rootkit. The malware most likely to have a human touch, rootkits are installed by crackers (bad hackers) on other people's computers. The rootkit is designed to camouflage itself in a system's core processes so as to go undetected. It is the hardest of all malware to detect and therefore to remove; many experts recommend completely wiping your hard drive and reinstalling everything fresh.

9. Keyloggers. No prize for guessing what this software does: yes, it logs your keystrokes, i.e., what you type. Typically, the malware kinds of keyloggers (as opposed to keyloggers deliberately installed by their owners to use in diagnosis computer problems) are out to log sensitive information such as passwords and financial details.

10. Dialers. Dialers dial telephone numbers via your computer's modem. Like keyloggers, they're only malware if you don't want them. Dialers either dial expensive premium-rate telephone numbers, often located in small countries far from the host computer; or, they dial a hacker's machine to transmit stolen data.

11. URL injectors. This software "injects" a given URL in place of certain URLs when you try to visit them in your browser. Usually, the injected URL is an affiliate link to the target URL. An affiliate link is a special link used to track the traffic an affiliate (advertiser) has sent to the original website, so that the original website can pay commissions on any sales from that traffic.

12. Adware. The least dangerous and most lucrative malware (lucrative for its distributors that is). Adware displays ads on your computer. The Wikipedia entry on malware does not give adware its own category even though adware is commonly called malware. As Wikipedia notes, adware is often a subset of spyware. The implication is that if the user chooses to allow adware on his or her machine, it's not really malware, which is the defense that most adware companies take. In reality, however, the choice to install adware is usually a legal farce involving placing a mention of the adware somewhere in the installation materials, and often only in the licensing agreement, which hardly anyone reads.

Are you ready to take on this dirty dozen? Don't go it alone. Make sure you have at least one each of antivirus and antispyware.