Trojan horses are software you have to manually install, but when you finish it has run a program that installed itself without your knowledge. You are tricked into running these threats with harmless looking emails that contain the trojan horse. You might not even know when you did it since it can hibernate and start anytime in the future.

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.

There are many 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. Most trojan horses now days are asking you to open an email or allow a pop up that is disguised as something you might be interested in. Maybe a friend invitation to Face Book or similar harmless seeming email could be the start of a very bad time for your computer.

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. They are failry simple to detect and stop with good Internet security software.