Hi,
Backdoor.Trojan is a detection name used by Symantec to identify malicious software programs that share the primary functionality of enabling a remote attacker to have access to or send commands to a compromised computer.
As the name suggests, these threats are used to provide a covert channel through which a remote attacker can access and control a computer. The Trojans vary in sophistication, ranging from those that only allow for limited functions to be performed to those that allow almost any action to be carried out, thus allowing the remote attacker to almost completely take over control of a computer.
I would suggest to refer technical write up of Backdoor.Trojan.
http://www.symantec.com/security_response/writeup.jsp?docid=2001-062614-1754-99&tabid=3