What are backdoor programs?

Backdoor is a general term for a malicious program that listens for commands on a certain network port. Most backdoors consist of a client component and a server component. The client component resides on the attacker's remote computer, and the server part resides on the infected system. When a connection between client and server is established, the remote attacker has a degree of control over the infected computer. For example, a backdoor may allow a attacker to monitor or take control of an infected computer, stealing data from it, uploading and activating viruses, or erasing user data and so on.