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 simply trojan) is any malware that misleads users of its true intent The term is derived from the Ancient Greek story of the deceptive Trojan Horse that led to the fall of the city of TroyTrojans are generally spread by some form of social engineering, for example where a user is duped into executing an email attachment disguised to appear not suspicious, (eg, a routine form to be filled in), or by clicking on some fake advertisement on social media or anywhere else Although their payload can be anything, many modern forms act as a backdoor, contacting a controller which can then have unauthorized access to the affected computer
Trojan_horse_(computing) - Wikipedia, the free encyclopedia