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.
CodeRed is an Internet worm that replicates between Windows 2000 servers running Microsoft's IIS (Internet Information Services) and the Microsoft Index Server 2.0 or the Windows 2000 Indexing Service. It does this by exploiting a bug known as "Unchecked Buffer in the Index Server ISAPI Extension," described by Microsoft in the Microsoft Security Bulletin MS01-033, released on June 18th, 2001