In computer and network security terminology, a layered defense is used to describe a security system that is build using multiple tools and policies to safeguard multiple areas of the network against multiple threats including worms, theft, unauthorized access, insider attacks and other security considerations. A layered defense ultimately should be able to provide adequate security at the following levels: system level security, network level security, application level security, and transmission level security. Also called a multi-layered defense.
See Intrusion Detection and Prevention in the Did You Know? section of Webopedia.
Also see The Difference Between a Virus, Worm and Trojan Horse in the Did You Know? section of Webopedia.