What is a trojan horse?

A Trojan Horse is a program that does something undocumented that the programmer intended, but that some users would not approve of if they knew about it. A very important distinction between Trojan horse programs and true viruses is that they do not replicate themselves. Trojans contain malicious code that when triggered cause loss, or even theft, of data. For a Trojan horse to spread, you must, invite these programs onto your computers–for example, by opening an email attachment or downloading and running a file from the Internet. Finally, despite the definitions, many people use the term “Trojan” to refer only to *non-replicating* malicious code, so that the set of Trojans and the set of viruses are disjoint.