Trojan, Trojan horse
A trojan (or Trojan) horse (or simply trojan) is a broad term for malicious software used for various nefarious purposes, often either destructive or for purposes of data theft. The determinative difference between a trojan and a virus or worm, is that it is not able to replicate or infect files on its own. Therefore it must rely on other means to get onto victims’ computer systems – for example, Internet drive-by-downloads, by exploiting software vulnerabilities or by social engineering, being downloaded by another type of malware (e.g. a worm, virus, or trojan-downloader), and so on. Its name, derived from the story of the conquest of Troy by smuggling soldiers into the city inside a massive wooden horse, suggests that it disguises its true function, often employing social engineering to achieve being executed.
Trojans are currently the most common malware type, used to open backdoors, exfiltrate user data (send it to a remote attacker), download other (malicious) software onto the infected system, and so on.