Definition:

A worm is a form of malware, or malicious software. A worm is similar to a virus, but unlike a virus, it doesn't need any help from humans to spread from computer to computer. Worms can replicate themselves and then propagate themselves to as many computers as possible. A worm might use your address book to harvest unsuspecting targets to receive emails with infected file attachments, or it might connect to other computers via a network.

Examples: Although viruses get all the press, worms are actually more insidious, because unlike viruses, worms are capable of spreading on their own, without any help from humans.