Virus-like program that spreads automatically to other computers by sending itself out by email or by any other means. A program that propagates itself by attacking other machines and copying itself to the affected machine.
Worms have self-replicating code that travels from machine to machine by various means. A worms first objective is merely propagation. Worms can be destructive depending on what payload they have been given. Worms may replace files, but do not insert themselves into files.
NextDoor is a worm that is installed by clicking an infected .zip file in a MSN chat client. Once this .zip is on the PC, it has the ability to connect to an IRC channel in order to download the Carlson Dialer and make long distance calls.