What is a Computer Worm?

A computer worm is a software program that is designed to copy itself from one computer to another, without human interaction. Unlike a computer virus, a worm can copy itself automatically.

Worms can replicate in great volume. For example, a worm can send out copies of itself to every contact in your e-mail address book, and then it can send itself to all of the contacts your contact’s e-mail address books.

Some worms spread very quickly. They clog networks and can cause long waits for you (and everyone else) to view Web pages on the Internet.

Courtesy :- Microsoft Corporation
http://www.microsoft.com/security/worms/whatis.aspx