Answer Description
A Trojan Horse is a malicious program that disguises itself as a useful, harmless program. For example, a free instant messaging program that records keyboard input to obtain passwords, credit card numbers, etc would be considered a Trojan Horse.
Wikipedia
In computing, a Trojan horse, or Trojan, is any malware which misleads users of its true intent. The term is derived from the Ancient Greek story of the deceptive wooden horse that led to the fall of the city of Troy.Trojans are generally spread by some form of social engineering, for example where a user is duped into executing an e-mail attachment disguised to appear not suspicious, (e.g., a routine form to be filled in), …
Trojan horse (computing) - Wikipedia, the free encyclopedia