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.
The link looks like a website or jpg, usually propogates via IM, but may be web based in some variants. Once you click the link, the worm will be downloaded to your computer and attack MSN messenger replacing it with another file.
The worm will send it self to all contacts on target's MSN list, and then download and install numerous malware and spyware applications- notably dollar revenue.