Wikipedia has great descriptions of trojans: http://en.wikipedia.org/wiki/Trojan_horse_(computing) and computer viri.

one is wearing a rubber and the other isn’t.

I may need to get some more often
I read this question all wrong_:)

A trojan is a virus hidden inside of another program.

yeah, basically a trojan tricks you into down loading it. like, click here and win now! or install this great new anti virus program for free! or, here are free cd keys for games you want to hack! like the trojan horse right?

Exactly KidKyle, that’s where the name came from, the torjan horse.

Yeah, I checked wikipedia already, obviously, and it answer left me still wondering. Don’t all viruses come disguised as something else? And how does a program detecting malware tell the difference between a virus and a trojan?

Well a virus can be a link saying “Click here to download some sexy pr0nz!”, and when you click it, it downloads a virus. A trojan virus would be a real program that you download and the virus is inside of that program, and a lot of the times you don’t find out it’s there until it’s too late.

Try this website for the Answer.
http://www.virushackerfreeinanhour.com/trojan_horse_virus.htm