Trojan horses are software you have to manually run, but do something malicious.

Some of them are just practical jokes handed out to friends, like one that appear to mess with your computer's vertical hold, making the screen roll like an old television set, or the ones that pretended to format your hard drive.

But there are malicious trojan horses, too. An infamous one was the Dialer.Gola, which was an email that pretended to be porn but was actually a program that changed your internet connection to a long distance dialup.

Because trojan horses do not replicate by themselves, they are trackable back to the sender. The person who handed it to you usually knew it was a trojan horse. This is why malicious trojan horses are spread via forged email, or download sites operated from non-cooperative countries, or via p2p sharing, which is delibrately hard to backtrack.