“Trojan horse”; a reference to the story of the Trojan War, a piece of malware (such as a virus program) contained within a legitimate program. Once your computer becomes infected by the worm or virus, it can be very difficult to repair the damage. Trojans usually come attached to another file, for example: .avi, .exe, or even .jpg. Many people do not notice or see file extensions, so what may appear as "fun program.zip" in reality could be "fun program.zip.exe." The difference here is the added .exe extension. Once the file is opened, the Trojan quietly installs itself.