Virus - Singular computer program that infects specified programs and hardware much like a biological virus. Needs a host like a conventional virus and can greatly damage the computer's memory by embedding its code into the computer's. This can lead to Trojans that can infect other computers.

Trojan - A program much like a virus that is disguised or hidden within a legitimate looking computer program, much like the Greek war tactic. However, the program needs to be triggered by the user much like the Greeks accepted the ill-fated gift. Trojans can change into other viruses in which the user needs to run again like the original Trojan in order for it to become infected

Worm - Worms are like viruses, however they do not need to infect a host program. They make copies of themselves and spread themselves across a computer network.