The Trojan horse, also known as trojan, in the context of computing and software, describes a class of computer threats (malware) that appears to perform a desirable function but in fact performs undisclosed malicious functions that allow unauthorized access to the host machine, giving them the ability to save their files on the user's computer or even watch the user's screen and control the computer.
Trojan Horses (not technically a virus) can be easily and unwittingly downloaded. For example, if a computer game is designed such that, when executed by the user, it opens a back door that allows a hacker to control the computer of the user, then the computer game is said to be a Trojan horse. However, if the computer game is legitimate, but was infected by a virus, then it is not a Trojan horse, regardless of what the virus may do when the game is executed. The term is derived from the classical story of the Trojan Horse.

Example

A program named "waterfalls.scr" serves as a simple example of a Trojan horse.
The author claims it is a free waterfall screen saver. When running, it instead unloads hidden programs, scripts, or any number of commands without the user's knowledge or consent. Malicious trojan horse programs conceal and drop a malicious payload on an affected computer.

By Fendy@xtechnology