What is trojan backdoor ?

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.

A computer with a sophisticated back door program installed may also be referred to as a "zombie" or a "bot". A network of such bots may often be referred to as a "botnet". Botnets have been well publicized in the news over the years, with different instances being given specific names such as Kraken, Mariposa, or Kneber, along with claims of hundreds of thousands of nodes belonging to certain networks.