A virus made by the popular condom company. I kid, I kid. Although there are many definitions for what a Trojan Horse constitutes, the most universal definition is a program that acts maliciously on your computer while posing as another program. Whereas viruses and worms sometimes replicate themselves, a Trojan Horse does not. Virus programs like McAfee can detect Trojan Horses and take ‘em out. You can avoid them by not downloading any program from a disreputable site, especially programs from peer-to-peer services like Kazaa or Limewire.
Worms
Worms infest themselves on your hard drive, then replicate themselves thousands of times and attempt to spread to other computers. Common worms will replicate themselves until they completely fill your hard drive, occasionally overwriting files in the process. Unlike viruses, they are not included with other programs or documents, but are independent. Worms are a very common problem, and often send themselves out to your list of email contacts in an exact replica of the bugged email that you opened. So, not only did you give a computer worm to all of your friends and family, they know that you opened an email titled, “STEAMING HOT FIREMAN CALENDAR!”. As Mastercard would say, priceless. The same process used for avoiding viruses can be applied to worms; get antivirus software and avoid opening emails with weird or suggestive titles. Worms tend to spread really well at companies, so if you have any doubts about an email from someone at your work, give ‘em a quick call to make sure that it was them that sent you the file. If you get a worm or need to test a file for worms, antivirus software will work, but make sure you have the most recent updates for your antivirus software in case the file you're checking has a worm that was recently created.