A Trojan Horse is a program that either pretends to have, or is described as having, a set of useful or desirable features, but actually contains a damaging payload. True Trojan Horses are not technically viruses, since they do not replicate. Many viruses and worms, however, use Trojan horse tactics to initially infiltrate a system. Although Trojans are not technically viruses, they can be just as destructive.
Many people use the term Trojan to refer only to non-replicating malicious programs, thus making a distinction between Trojans and viruses.