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