A Trojan or Trojan horse is a malicious program that is hidden inside another useful application. Trojans are seemingly harmless programs that hide malicious programs such as a key logger that could capture all keystrokes including passwords, without the knowledge of the user. The term Trojan Horse has originated from the hollow wooden statue of a horse in which a number of Greeks are said to have concealed themselves in order to enter and conquer the ancient city of Troy.