Trojan Horses
"Trojan Horses" are named after the Greek legend of the Trojan Horse. The Greeks, at war with the City of Troy, had been unable to get beyond the walls to enter the city. So they built a large wooden horse, and hid a some of their soldiers inside. They pretended that the horse was a gift to the Trojans. When the people of Troy saw the horse, they opened the gates and moved the horse into their city. At night the Greeks came out of the wooden horse and opened the city gates. The Greek army stormed into the city and Troy was defeated.Similar to the Greek legend, we define a Trojan Horse as a computer file or program which seems to be useful, needed, or wanted, but which is, in actual fact, a harmful file or program. A Trojan horse looks like it is doing something innocent, such as showing a picture inside an e-mail or installing a screen saver. But in fact it is also doing something else - such as erasing files, or secretly sending information stored on your computer (such as passwords and other personal information) to the hacker who wrote it.
One key factor which makes a Trojan (as it is called for short) different from other malware is that the person receiving the file or program has to activate it before it can begin to operate. In order to get the user to activate it, the creator of the Trojan might disguise it as a screen saver and offer it on the web as a free screen saver. When one downloads the screen saver and activates it on one's computer, one sees a nice screen saver. But meanwhile, in the background, invisible to you, another program starts running and starts sending your personal data to some other site.
There are many ways to disguise such a harmful program. Common examples are: free games, screensavers, free tools for your computer, free videos or music files, free pictures of barely dressed females (which some males find really hard to resist:) and so on.
So a Trojan is called a Trojan, because, for it to work, it has to trick the person receiving it into activating the program and running it on their computer. Children and teenagers are particularly vulnerable to this sort of malware, as they are easy to entice with "free" games or cool videos. They are also least likely to be informed about the ramifications of Trojans.