A computer worm is a self-replicating computer program, similar to a computer virus. The main difference between the two is that a computer virus attaches itself to, and becomes part of, another executable program, while a worm is self-contained; it does not need to be part of another program to propogate itself. In addition to replication, a worm may be designed to do any number of things, such as delete files on a host system, or send documents via email.
The first worm, known as the Morris Worm, was written by Robert Morris at the MIT Artfical Intellegence Laboratory. It was released on November 2, 1988, and quickly infected a great many of the computers on the Internet. It propagated through a number of bugs in BSD Unix and its derivatives. Morris himself was convicted under the US Computer Crime and Abuse Act, received 3 years probation, community service and a fine in excess of $10,000.