Trojan horse

Initially, a Trojan horse meant a program masquerading as a normal one to perform a given task, sometimes even bearing a known name (a sort of disguise), but which, once installed, carried out harmful action totally different from its "official" function.

The term currently means almost any program that installs itself fraudulently (often via e-mail or a malicious web page) to perform a hostile task unknown to the user. Harmful functions include spying, massive spam attacks, opening access to a hacker etc.

The difference between Trojan horse, spyware, keylogger, back door is often only a matter of words or context.