Virus A computer virus is a program created specifically to invade computers and networks and wreak havoc on them. The mischief caused can be very minor, such as causing a funny image or cryptic message to be displayed on your screen, or it can do some serious damage by altering or even destroying files.

Trojan Horse A Trojan Horse is a seemingly-innocent program that contains and conceals harmful code. When a Trojan Horse is opened, the malicious code performs its damage on the unsuspecting computer.

Worm A software program capable of reproducing itself that can spread from one computer to the next over a network. Worms take advantage of automatic file sending and receiving features found on many computers

Adware A program that displays banners while being run, or reports users habits or information to 3rd parties. Often these programs actively deliver pop up adverts to your PC.

Spyware A general term for a program that surreptitiously monitors your actions. While they are sometimes sinister, like a remote control program used by a hacker, software companies have been known to use spyware to gather data about customers. The practice is generally frowned upon.