What is a Trojan horse virus for computers?

Answer 1

When downloading Trojan into your PC, it would be would present itself as a useful software at the start. This will literally damage your computer as soon as it has been installed into the system. Some people would receive and start installing the software; this is tricking the user into opening the program. What this does is, it could automatically change your desktop, putting unnecessary icons up. This software can fatally affect your computer by deleting files and damaging your files and folders on your system.

The term Trojan Horse for malicious software comes from the legend of the Battle of Troy. The city was impenetrable, so the enemy could not get in the conventional way. The gates were guarded and the guards would never knowingly let enemies inside. So the enemies got clever. They created a huge horse statue which they hid inside, and the city thought it was a gift and brought it inside. Then the enemy soldiers climbed out of the statue at the right moment and started killing. Then they probably opened the gates and let the rest of their men inside. So just like this legend, malicious software authors would create something that seems useful or entertaining, and the users would run or install it and either lose files or get infected with a computer virus.

Answer 2

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.

source: Wiki