Virus - is a small computer program written to alter the way a computer operates. It should meet two criteria
1.) It must execute itself.
2.) It must replicate itself.
Worms - Worms are programs that replicate themselves from system to system without the use of a host file.This is in contrast to viruses, which requires the spreading of an infected host file.
Trojans - Trojan horses contain malicious code that when triggered cause loss, or even theft, of data.