A worm, like a virus, is a type of malware that moves from computer to computer (most commonly through networks) and can cause malicious problems. Unlike virus, a worm can stand alone - it is a complete program on itself. It basically does not need to depend on other files before it unleashes its potentials. In other words, it could operate more or less independently of other files. Also, unlike virus, a worm may not cause a terrible harm to a network. It could just be something less serious as just consuming bandwidth.
