What is a Trojan Horse?

Trojan Horse:

A trojan horse is not a virus. Trojan horses are designed to allow a hacker remote access to a target computer system. It is a program that you run because you think it will serve a useful purpose such as a "game" or "free software". Like a "trojan horse" it serves not as it claims but facilitates unauthorized access to the user's computer system, in order to damage files or perhaps plants a virus into your computer. Imagine you have a fence around your house and someone came and put a hole in it. That's pretty much what a trojan does, it's a bit more involved but the hole (open port) is what allow hackers to get in.

A trojan horse does not replicate or spread like a virus. Most virus checking programs detect trojan horses, ideally preventing them from ever being installed.

The hacker need not be the individual responsible for distributing the Trojan horse. It is possible for hackers to scan computers on a network using a port scanner in the hope of finding one with a Trojan horse installed.