Trojan Horse is a destructive program that masquerades as a benign application. Unlike viruses, Trojan horses do not replicate themselves but they can be just as destructive. 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.
The term comes from the a Greek story of the Trojan War, in which the Greeks give a giant wooden horse to their foes, the Trojans, ostensibly as a peace offering. But after the Trojans drag the horse inside their city walls, Greek soldiers sneak out of the horse's hollow belly and open the city gates, allowing their compatriots to pour in and capture Troy.
Types of Computer Trojan:
Remote Access Trojan: Abbreviated as RATs, a Remote Access Trojan is one of seven major types of Trojan horse designed to provide the attacker with complete control of the victim's system. Attackers usually hide these Trojan horses in games and other small programs that unsuspecting users then execute on their PCs.
Data Sending Trojan: A type of a Trojan horse that is designed to provide the attacker with sensitive data such as passwords, credit card information, log files, e-mail address or IM contact lists. These Trojans can look for specific pre-defined data (e.g., just credit card information or passwords), or they could install a keylogger and send all recorded keystrokes back to the attacker.
Destructive Trojan: A type of Trojan horse designed to destroy and delete files, and is more like a virus than any other Trojan. It can often go undetected by antivirus software.
Proxy Trojan: A type of Trojan horse designed to use the victim's computer as a proxy server. This gives the attacker the opportunity to do everything from your computer, including the possibility of conducting credit card fraud and other illegal activities, or even to use your system to launch malicious attacks against other networks.
FTP Trojan: A type of Trojan horse designed to open port 21 (the port for FTP transfer) and lets the attacker connect to your computer using File Transfer Protocol (FTP).
Security Software Disabler Trojan: A type of Trojan horse designed stop or kill security programs such as an antivirus program or firewall without the user knowing. This Trojan type is normally combined with another type of Trojan as a payload.
Denial-of-Service Attack Trojan: Short for denial-of-service attack, a type of attack on a network that is designed to bring the network to its knees by flooding it with useless traffic. Many DoS attacks, such as the Ping of Death and Teardrop attacks, exploit limitations in the TCP/IP protocols. For all known DoS attacks, there are software fixes that system administrators can install to limit the damage caused by the attacks. But, like viruses, new DoS attacks are constantly being dreamed up by hackers.