Trojan Horses are closely related to computer viruses, but they differ in that they do not attempt to replicate themselves. More specifically, a Trojan Horse performs some undesired — yet intended — action while, or in addition to, pretending to do something else. A common example is a fake login program, which collects account information and passwords by asking for this info just like a normal login program does. One of the most insidious types of Trojan horse is a program that claims to rid your computer of viruses but instead introduces viruses onto your computer.
from www.scambusters.com