A computer worm is a kind of malware that is able to copy itself, and spread. Unlike a computer virus, this is done without changing files on a computer, and without affecting the boot sector of media. Worms do not need human help to reproduce. Worms spread over networks, or on removable media like USB flash drives. They almost always need a helper program that is able to access the network, or that executes when a new medium is inserted. In that way, a Worm could start a program to send electronic mail, and send itself on to all the addresses in the address book of the email application. A very famous worm that appeared in 2000 was called ILOVEYOU (or Love Letter), it sent a simple mail with the subject ILOVEYOU, and an attachment of a document that contained the worm code.
Worms usually do not cause direct damage to a system. But they consume memory and network bandwidth, slowing down the computer. This makes it almost impossible to work with. Other examples of Worms are CodeRed and Sircam.