Since the first documented computer virus in the late 60s, volumes have been written on the subject and it's a billion dollar/year industry. A recent search of AltaVista for computer virus produces more than 882,000 results. This article's intent it to provide you with basic knowledge of virus/trojans, simple tips to reduce the chances of getting one, and some great links for additional information.
Virus: A small program that is loaded on your machine, normally without your knowledge, that usually can duplicate itself, infecting legitimate programs with its program, making it difficult to remove. The intent of virus' vary from annoyances to destructive payloads.
Trojan: An apparently harmless program containing malicious programming that allows the unauthorized collection, falsification, or destruction of data. Internet trojans normally contain a 'backdoor' so that others may gain unauthorized access to the infected PC.
Worm: Worms are very similar to viruses in that they are computer programs that replicate themselves and that often, but not always, contain some functionality that will interfere with the normal use of a computer or a program. The difference is that unlike viruses, worms exist as separate entities; they do not attach themselves to other files or programs. A worm can spread itself automatically over the network from one computer to the next. Worms take advantage of automatic file sending and receiving features found on many computers.

The most widely publicised virus was the Melissa Virus, a MS Word-based macro that rapidly duplicated itself through email, causing mail servers to get bogged down. Common viruses/worms include the Happy99, Hybris, MTX, Kak, Klez, IFrame Exploit, Navidad, Nimda, and Magistr. Common trojans include Back Oriface, NetBus, Sub7, DeepThroat, and theThing. Many variations of the orignals have been produced and are being distributed.

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