Trojan Horses, Backdoor Viruses and to a certain extent, Worms, are all examples of Malware, or Malicious Software.

Trojan Horses, or simply Trojans, are viruses that are disguised or hidden in legitimate software. You can install a Trojan without even knowing it.

A Trojan Horse gets its name from the Greek myth of the Trojan war. The Greeks gave a giant wooden horse statue to the Trojans as a gift. Little did the Trojans know there were Greek soldiers hiding inside the horse, and once it was inside the gates of Troy, the Greeks leapt out and slaughtered the Trojan army unaware.

Unlike worms, a Trojan must be installed by the user before it can replicate and spread.

A Backdoor is not actually a virus, but a method viruses use to spread and install themselves through an open port on your computer. A backdoor is what viruses use to bypass security or gaining remote access to your computer while remaining hidden. Your computer then becomes a zombie and is used to spread spam, viruses and other fraud.

50% to 80% of spam is now sent by zombie computers, unbeknownst to their owners. If your computer is infected with a Trojan that is spreading spam emails through a Backdoor, you a liable for covering the bandwidth costs and you may even get your ISP account canceled for violating anti-spam laws.

It is very important that your Antivirus software scans for Trojans and open Backdoors. This is also the job of a good Firewall, which monitors your computers ports and detects unusual activity.