A Trojan or Trojan Horse
A Trojan or Trojan Horse is a program that appears legitimate, but performs some malicious and illicit activity when it is run. It may be used as a hacker tool to leak a user's password information or it may simply destroy programs or data on the hard disk. Contrary to a virus, a Trojan does not replicate itself. It stays in the computer doing its damage or allowing somebody from a remote site to take control of the computer. Trojans often sneak in by attaching to a free game or other utility.