A computer worm is similar to a virus and is also a form of Malware.
Worms spread from computer to computer, but unlike a virus, it has the capability to travel without human action.
A worm uses the file or information transport features on your system and this is what allows it to travel unaided.
Because it is capable of replicating itself on your system, rather than your computer sending out a single worm, it could send out hundreds to thousands of copies of itself which would have a huge devastating effect.
One good example would be for a worm to send a copy of itself to everyone listed in your e-mail address book.
Then, the worm replicates and sends itself out to everyone listed in each of the receiver's address book, and the problem continues .
Worms get their name from their action of worming their ways into the other files to disrupt them.