Trojan Horse virus is a self-contained program that appears to be legitimate, but in fact does something malicious. Trojans do not infect other files as viruses do, nor do Trojans make copies of themselves as worms do.           

A Trojan virus can be very malicious indeed. Most of them are intent on controlling your computer. These are called Remote Access Trojans, or RATs for short. If someone has placed a Trojan on your PC, they'll be able to see everything that you can. Some of them can even controll your webcam.

If that weren't bad enough, the attacker will have access to your PC, enabling him to upload nasty things to your computer. After all, why should he store these things on his computer when he has access to yours?

Most Trojans these days, though, are placed on your computer by criminals. If you type your account details in to a website, for example, then the attacker can record what you type. If a criminal has controll of a lot of computer, he could also launch something called a Denial of Service attack. A DoS attack is when a lot of malicious computers attack a particular network or website. The network has so many request that it can't cope, so has to shut down. The criminals then blackmail the owner ("We'll let you have your site back if you give us money".) Many gambling sites have been hit by this type of attack.

• Remote Access Trojans : controls your computer from remote locations.
• Data Sending Trojans : used to sensitive data such as passwords, financial information to hackers.
• Destructive Trojans : used to corrupt or delete files on your computer.
• Pr0xy Trojans : uses your computer to send spams, financial fraud & other illegal activities.
• FTP Trojans : used to open port 21 (FTP port) and lets the attacker connect to your computer.
• Security software disabler Trojans : disables security software's such as an antivirus program or firewall.
• Denial-of-service attack (DoS) Trojans : used to shut down a website, server or network.