1. A virus that's designed to find all data in memory or on disk and alter any data it encounters. The alteration may be to change certain chracters to numbers or to swap bytes of stored memory. A few programs may still run, but usually data is irretrievably corrupted. From QUECID

2. A worm is a virus that does not infect other programs. It makes copies of itself, and infects additional computers (typically by making use of network connections) but does not attach itself to additional programs; however a worm might alter, install, or destroy files and programs. From Matisse

3. A program that propagates itself by attacking other machines and copying itself to them. Example: In the late 1980s, the Morris Worm shutdown the Internet for a couple of days. At the time, well-known bugs in the UNIX sendmail program could allow a hacker to break into machines. Robert T. Morris wrote a program that would san machines for these security holes, then break into the machine. After breaking in, the program would copy itself up to that machine, then launch it. In this manner, the worm spread from machine to machine, multiplying until it had broken into nearly every machine which contained these bugs. However, the worm itself had a bug where it couldn't detect that a machine had already been broken into. Therefore, it would repeatedly break into the same machine over and over, until it machine collapsed from running too many instances of the worm. Copycats of the Morris Worm pop up repeatedly as new security holes appear in popular systems (like Linux), but they never have the devastating effect of the Morris Worm. Example: In the late 1999, the Melissa Worm/Virus nearly disabled the Internet. The worm spread by e-mailing itself to the first 50 people in a user's e-mail address book. Victims would then receive an e-mail from somebody they knew and trusted, so they would open the attached document and run the macros. In this manner, Melissa spread from inbox to inbox. Melissa is sort of a cross between a virus and a worm: it had the ability to spread itself like a worm, but it still required user interaction. Example: Around 1998, the ADMworm traveled by exploiting a few well-known Linux vulnerabilities in Linux machines, breaking into the machine, installing itself, then hunting for more machines. Example: Having failed to learn their lesson in 1999, the industry was pummeled by the ILOVEYOU worm in early 2000. It spread in much the same way, though this time it was a VBS script rather than an .exe. Contrast: There really is not difference between a worm and a virus. The dividing line is usually drawn along the amount of human interaction involved, and how it spreads from machine to machine. A worm spreads itself with zero human interaction, whereas a virus is spread by human contact: humans exchange files from machine to machine, and when a human runs the infected program, the virus only infects other files on the same machine. Some viruses do attack servers, but only because the user is connected to the server. The Melissa Virus/Worm crosses the line: it spreads from one machine to another like a worm, but it must be launched by the user like a virus. Example: The SirCam Worm of July 2001 didn't explode as fast as other viruses, but had a large effect on the net through its "background" operation on a machine and the technique of spidering the cached files in web-browsers looking for e-mail addresses. From Hacking-Lexicon

4. n. [from `tapeworm' in John Brunner's novel "The Shockwave Rider", via XEROX PARC] A program that propagates itself over a network, reproducing itself as it goes. Compare virus. Nowadays the term has negative connotations, as it is assumed that only crackers write worms. Perhaps the best-known example was Robert T. Morris's Great Worm of 1988, a `benign' one that got out of control and hogged hundreds of Suns and VAXen across the U.S. See also cracker, RTM, Trojan horse, ice. From Jargon Dictionary