What is a Worm?
Worms operate differently. They work in a way such that they keep replicating themselves over and over in your system.
Worms generally come through our email client, but people can also get infected if they accept a Trojan File which has as the payload a worm. If you receive a worm program through your email, and then execute it, this program sends the worm file out to every email address listed in your email address book. If you work in a major corporation, this could means hundreds of people, and so the multiplying continues.