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.
In computing, a Trojan horse 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 Troy.Trojans generally spread by some form of social engineering; for example, where a user is duped into executing an email attachment disguised to appear innocuous (e.g., 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 who can then have unauthorized access to the affected computer. Ransomware attacks are often carried out using a Trojan.
Unlike computer viruses and worms, Trojans generally do not attempt to inject themselves into other files or otherwise propagate themselves.