Viruses - A computer virus is a computer program usually hidden within another seemingly innocuous program that produces copies of itself and inserts them into other programs or files, and that usually performs a malicious action (such as destroying data).

Worms - are very similar to viruses in that they are computer programs that replicate functional copies of themselves (usually to other computer systems via network connections) and contain some functionality that will interfere with the normal use of a computer or a program. The difference is that unlike viruses, worms exist as separate entities; they do not attach themselves to other files or programs. Because of their similarity to viruses, worms are often also referred to as viruses.

Trojan horse - Named after the wooden horse the Greeks used to infiltrate Troy, a Trojan horse, or more simply Trojan, is a piece of software which appears to perform a legitimate action but in fact performs an illicit action, such as a computer virus. Trojan horses are notorious today for their use in the installation of backdoor programs, allowing an unauthorised individual unlimited access to your computer.