QUOTE: There are seven major types of Trojan:

1. The remote access Trojan takes full control of your system and passes it to the hacker. This Trojan acts as a server and usually listens on a port that is not readily available to other attackers (there are over 130,000 ports available). While a firewall will block external hacker access to the Trojan (only if the "unused" ports have been blocked by your up-to-date firewall configuration), an internal hacker (one who resides on the agency side of the firewall) can connect to this type of Trojan without any trouble.

2. The data-sending Trojan sends data back to the hacker. These Trojans can communicate with the originating hacker by means of e-mail to a Web-based e-mail account or Web site. Key-loggers (programs that log and transmit each keystroke you make) are common data-sending Trojan payloads.

3. The destructive Trojan has only one purpose: to destroy and delete files. The Trojan can be activated by the hacker, or written to execute at a specific time/date. These Trojans are different from your garden-variety Trojans since they are written purposefully to attack YOU, and therefore unlikely to be detected by anti-virus software.

4. The denial-of-service (DOS) attack Trojans combine your computing power with that of the other computers/systems it infects to launch an attack on another computer system. By using the power of 300 or more infected computers, the hacker can flood a computer system with so much traffic that it crashes.

5. The proxy Trojans allows a hacker to turn YOUR computer into HIS server to make purchases with stolen credit cards, sell and trade child pornography, and run other organized criminal enterprises in YOUR name.

6. The FTP Trojan opens port 21 (the port for FTP transfer) and lets the attacker connect to your computer using File Transfer Protocol (FTP).

7. The security software disabler Trojan is designed to stop or kill security programs such as anti-virus software, firewalls, etc., without you knowing it. This Trojan type is normally combined with another type as a payload.

Just adding some usefull info.

Basics on trojans: http://netsecurity.rutgers.edu/trojan.htm