What is a Trojan?
A Trojan is a type of malicious software disguised or included with a legitimate piece of software. It's hiding inside. The reason we hide it is that it's easy to install. The easiest way to get something done is to have the user do it for us. When it comes to Trojans, some people get this confused.
The Trojan horse gets its background from Greek mythology about the Trojan War, where the Greeks attacked the city of Troy. At the end of the war, the Greeks came up with a final plan of attack—they would build a giant hollow wooden horse (and for some strange reason, they were sacred to Trojans), and the hollow horse would be filled with soldiers. The Trojans brought the horse into the city and when they went to bed, the soldiers came out, including Brad Pitt, all glistening, and they ransacked the city. That's basically the same concept here. We're going to have a legitimate program, but our Trojan will contain some type of spyware, keylogger...