According to legend, the Greeks won the Trojan War by hiding in a huge, hollow wooden horse to get into the fortified city of Troy. In today's computer world, a Trojan horse is a malicious, security-breaking program that is disguised as in the form of a
screen saver, a game, or some other valuable program. Trojans are executable programs, which means that when you open the file, it will run and perform an action(s). These programs can combine the risks of both the virus and the worms as it may be a true program running in the background. Unknown to you, a Trojan horse may be compiling an email message, listing the network configuration and sending it off to the hacker, all without your knowledge.

It is very common for Trojan horse programs to use simple games as a front-end of the overall program. In Windows, executable programs have file extensions like "exe," "vbs," "com," "bat," "pif," "scr," "lnk," or "js."