Trojan horse (computing)
A trojan horse (Sometimes called a trojan) is a kind of malware. It is special computer program that pretends to do a certain thing, but in reality it does something else, such as allow a stranger to access the computer and change it and read its information. In some cases the user notices, in other cases they do not. Spyware programs are current examples of programs that work as trojans.
It is rare to get a trojan on your computer by merely visiting a website, though it can happen. Usually, though, a trojan is attached to something you download. Usually they attach themselves to music, wallpapers, backgrounds, games, and especially executable files (.exe) and so on.