Backdoor is malicious software that the primary functionality of enabling a remote attacker to have access to compromised computer. A computer infected by backdoor – will named “bot”. So a network of such “bots” be referred to as a “botnet”.
Backdoors are used to provide a covert channel through which a remote attacker can access and control a computer. They allow almost any action to be carried out, thus allowing the remote attacker to almost completely take over control of a computer.