If ever you are the victim of a Trojan horse, you’ll know how the Trojans felt when they discovered that the Greeks led by Ulysses were hidden inside the wooden horse they had given to Troy as a gift. It was a poisoned chalice, and it cost them their city. On the web, a Trojan horse can destroy your computer with all your files, or, worse, it could raid your bank account.
The term Trojan horse was originally used to mean a pirate programme disguised as something else, for instance a useful software application with new user functionality. But once it is downloaded and installed, it executes commands that damage your computer, of which you, of course, know nothing. By extension, the term has come to mean almost any programme that installs itself on your computer by deception (often by using an email or website as bait) in order to do something damaging like sending spam, spying on your computer or gathering your confidential data (passwords, for instance) for use by the pirates. Trojan horses often infiltrate via hidden ports or entrances. Software manufacturers create hidden ports or "backdoors", sometimes more crudely known as "back orifices" to be able to carry out remote testing or maintenance on the software. The pirates use these concealed entrances to infiltrate your computer and take control of it.