A computer worm is a harmful program that differs from a virus in several respects.
First of all a worm is an independent program that can be found on the hard disk, unlike a virus, which hides as parasites in files or executable code on the boot sector of the disk. There are a few very rare worms that do not sit on the disk, and remain only in memory.
A worm can arrive directly through the network via an open port, but the traditional method is by an e-mail attachment. Some are executed directly when the e-mail is read (especially if the computer system has not been updated). But for most of them, the attachment has to be clicked for the worm to execute.
A worm does not multiply locally, unlike a virus; the usual method of propagation is to attach itself automatically to automatically generated e-mails. These e-mails are sent to various addresses without the knowledge of the user. The worm usually takes these addresses from files on the disk (especially the address book), or it builds semi-random addresses.
Worms usually install other harmful programs on the computer: spyware, keyloggers, back doors, Trojan horses. These programs can be used to spy on your activity, capture pass words or credit card numbers, or to take control of your computer remotely and transform it into a zombie PC. Such a PC can be used as a relay for denial of service attacks or for mass spam e-mailing. The term virus is used incorrectly to denote this malware by the general public, and even by some specialised articles.