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.
This is a worm that spreads itself via MSN Messenger. The infected machine will send messages to everyone on its contact list a message such as "holy ****, have a look at this!!!!!! www.windowslivemessenger.biz/m...". This site downloads contactinfo.scr.