A program that appears desirable to encourage downloading and/or running the program, but actually contains something undesirable, often in addition to the desirable program. The undesirable payload may install resident and harmful code on the host computer. A Trojan may, for example, contain a virus or worm. A Trojan is, however, itself not a virus or worm as it requires action be taking by the user to spread itself.
Trojans are often distributed masquerading as pirated copies of popular software.