You open your e-mail mailbox one day, and a person whom you hardly know has sent you the message: "I love you". Curious, you open the email, only to find an attachment in the email, which destroys most of the data on the hard-disk of your computer! The phrase "Trojan Horse" comes from Homer's Iliad. In the Trojan War, the Greeks presented the citizens of Troy with a large wooden horse in which they had secretly hidden their warriors. During the night, the warriors emerged from the wooden horse and overran the city. Similarly, in computer lingo, a Trojan Horse is a program that sneaks into your computer, and then seeks to destroy it.
A Trojan horse is a program in which malicious or harmful code is contained inside apparently harmless programming or data. Once it is allowed to execute, it can control your computer and do its chosen form of damage, such as ruining the file allocation table on your hard disk, or delete MS-Word files. In one celebrated case, a Trojan horse was a program that was supposed to find and destroy computer viruses. A Trojan horse can be considered a virus if it is widely redistributed.
Trojans are executable programs, which means that when you open the file, it will run and perform some action(s). In Windows, executable programs have file extensions like "exe", "vbs", "com", "bat", "pif", "scr", "lnk", or "js". Some actual trojan filenames are: "dmsetup.exe", "Movie.avi.pif", and "LOVE-LETTER-FOR-YOU.TXT.vbs". When there are multiple extensions, only the last one counts.
A precaution you can take to save yourself from trojan attacks is to use a feature to view the full file-names (including file-extensions), so that you can see what the real file name is. Keep track of news items about popular trojan attacks, and keep your anti-virus software updated. If you are working in a networked department, try to minimize share access to your computer, giving share permissions only when required. Finally, never accept executable files as attachments in e-mail.
Trojans can be spread in the guise of literally any other file, such as a game, a picture, mp3 song. You probably downloaded the trojan from a WWW or FTP archive, or from a chat site. Typically you must run the Trojan manually. You may have known it was an executable but thought it was something else, been fooled by a hidden file extension, or just gotten careless and clicked on it. Trojans usually do their damage silently over your disk or network. The first sign of trouble is often when others tell you that you are trying to send them some trojan!
