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.Netlip.Worm is a worm that attaches itself to email messages each time that you send email to anyone from an infected computer. The virus may trick a user into running it since it has Windows folder icon association - on a default system where extensions of known file types are hidden, the file will appear exactly like a Windows folder.