What is Trojan Horse?

A Trojan horse, or Trojan, is software that appears to perform a desirable function for the user prior to run or install, but steals information or harms the system

Trojan horses may allow a hacker remote access to a target computer system. Once a Trojan horse has been installed on a target computer system, a hacker may have access to the computer remotely and perform various operations, limited by user privileges on the target computer system and the design of the Trojan horse.

Courtesy :- http://en.wikipedia.org/wiki/Trojan_horse_(computing)