What's the difference bewteen 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, usually by some action performed on the part of the computer user such as clicking on a file, the virus lies in wait for a command or certain date to deliver its payload, sometimes harmless and other times devastating. Receiving or downloading files such as "AOL4FREE.exe" or free screen savers are more likely to contain viruses than not. "Trust everyone, but brand your cattle!"
A worm is a sub-classification of a virus. A worm contains a small computer program that can replicate itself, and like a worm, wiggles its way through a computer system or network until it finds a way onto the Internet. Many worms have shut down corporate america for days at a time. 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 worms generate. The computer user is usually no more the wiser that anything is even happening, when in fact once the user connects to the Internet, the worm goes into action e-mailing itself to the user's family, friends, collegues and any other e-mail address it came across on the user's system or network.