What's the difference between a "virus" and a "worm" ? :

     A virus is a computer program that attaches itself to a computer system by incorporating a small portion of data into other programs. Once triggered, sometimes by a certain date or sometimes by some action performed by the computer user such the click of the mouse, the virus lies in wait for the command to deliver its payload, sometimes harmless and sometimes devastating. Receiving files such as "AOL4FREE.exe" are likely to contain such viruses. Such a program is usually harmless until clicked on, at which time is becomes active on the user's computer system.

     A worm is a small computer program that can replicate itself, and like a worm, wiggle its way through a computer network until unleashed onto the Internet. Many have shut down corporate america for days at a time trying to remove these types of worms. Microsoft, IBM, and other large businesses have had their e-mail systems literally collapse due to the volume of incoming and outgoing e-mail that these types of worms generate trying to breed into the wild. The computer user is usually no more the wiser that anything is happening, when in fact the worm just caused their computer to e-mail itself to the user's family, friends and colleagues.