Official description:
"A program that propagates itself by attacking other machines and copying itself to them. Both worms and viruses are self-replicating code that travels from machine to machine by various means. Both worms and viruses have, as their first objective, merely propagation. Both can be destructive, depending on what payload, if any, they have been given. But there are some differences: worms may replace files, but do not insert themselves into files. In contrast, viruses insert themselves in files, but do not replace them."