Trojan = A destructive program that masquerades as a benign application. Unlike viruses, Trojan horses do not replicate themselves, but they can be just as destructive. One of the most insidious types of Trojan horses is a program that claims to rid your computer of viruses but instead introduces viruses onto your computer.

Trojans can slow down your computer and interfere with your Internet Access. Some Trojans will search your computer for passwords and send the passwords to some hacker on the Internet. Others will allow the hacker into your computer to look at or steal some of your files or remotely control your computer in some way.

The term comes from a story in Homer's Iliad, in which the Greeks gave a giant wooden horse to their foes, the Trojans, ostensibly as a peace offering. But after the Trojans dragged the horse inside their city walls, Greek soldiers snuck out of the horse's hollow belly and opened the city gates, allowing their compatriots to pour in and capture the city of Troy.