Dealing with unwanted invasions of your machine
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 attack launched by having thousands of computers all try to connect to a web server at the same time.