Trojan - technical definition

A program that appears legitimate, but performs some illicit activity when it is run. It may be used to locate password information or make the system more vulnerable to future entry or simply destroy programs or data on the hard disk. A Trojan is similar to a virus, except that it does not replicate itself. It stays in the computer doing its damage or allowing somebody from a remote site to take control of the computer. Trojans often sneak in attached to a free game or other utility. For information about various Trojans that are spread on the Internet, visit the Lockdown Corporation at www.lockdowncorp.com. See rootkit, RAT, Back Orifice, NetBus, PrettyPark, Talking Trojan and virus.

The Trojan Horse
Trojan comes from Greek mythology, in which the Greeks battled the Trojans (people of Troy). After years of being unable to break into the fortified city, the Greeks built a wooden horse, filled it with soldiers and pretended to sail away. After the Trojans brought the horse into the city, the Greek soldiers crept out at night, opened the gates of Troy to the returning soldiers, and Troy was destroyed.