Bacteria

– program that consumes system resources by replicating itself

Logic Bomb

– logic embedded in a program that checks for a certain set of conditions to be present on the system. When these conditions are met, it executes some function resulting in unauthorised actions

Trapdoor

– Secret undocumented entry point into a program, used to grant access and authorization leading to security violation.

Trojan Horse

– Secret undocumented routine embedded within a useful program and is not useful for the program software.

Virus

– Code embedded within a program that causes of itself to be inserted in one or more other programs, and performs unwanted functions

Worm

– Program that can replicate itself and send a copies across network connections which is not useful.