Definitions:
Virus: is a self-replicating program which spreads by attaching itself to a host program. Viruses with a malicious payload cause damage to data, software or hardware.
Worm: is a program designed to replicate itself without human action. It disseminates complete copies of itself across computer networks or via email. THe worm itself may be destructive or it may do harm by consuming memory or bandwidth as it spreads, thus causing infected machines to stop responding.
Trojan Horse: is a malicious program wrapped in a benign exterior. Unlike viruses and worms, trojans do not replicate, but they can cause extensive damage to an infected system.
