A virus is a computer program that infects other programs. That's different from a worm, which simply propagates itself from computer to computer by *temporarily* using some other program. A Trojan is a program that pretends to be something it is not.
Think of it this way. Your pal Bill has a spare key to your house. A worm would either steal the key to let itself in (where it would then presumably find other keys to steal) or sneak in behind Bill while he was feeding your goldfish. A virus would take control of Bill's body and make him open the door. Bill would otherwise function normally, though he might twitch and fall down now and then, either because the virus wanted him to or just because it screws him up in other ways. A Trojan would be someone else dressed up to look like Bill who you then hand another spare key to. The fake Bill might then become a virus, but he started out as a Trojan.
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More Articles by Tony Lawrence © 2011-07-05 Tony Lawrence
Securing a computer system has traditionally been a battle of wits: the penetrator tries to find the holes, and the designer tries to close them. (Gosser)