Trojan horse, or Trojan, is any malicious computer program which is used to hack into a computer by misleading users of its true intent.
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 be unsuspicious, (e.g., a routine form to be filled in), or by drive-by download or from spam links and fake pop up & Advertisement. 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. This infection allows an attacker to access users' personal information such as banking information, passwords, or personal identity (IP address). Download and install other malware, such as viruses or worms. Unlike computer viruses and worms, Trojans generally do not attempt to inject themselves into other files or otherwise propagate themselves.
Source: Wikipedia, May 2017