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.
W32.Ticton.A, a mass-mailing worm harvests addresses from a number of sources, spreads over mapped network drives and accessible Windows shares, and can insert propagation code into existing HTML documents to spread through IIS.