Backdoors are designed to give malicious users remote control over an infected computer. In terms of functionality, Backdoors are similar to many administration systems designed and distributed by software developers.
These types of malicious programs make it possible to do anything the author wants on the infected computer: send and receive files, launch files or delete them, display messages, delete data, reboot the computer, etc.
The programs in this category are often used in order to unite a group of victim computers and form a botnet or zombie network. This gives malicious users centralized control over an army of infected computers which can then be used for criminal purposes.
There is also a group of Backdoors which are capable of spreading via networks and infecting other computers as Net-Worms do. The difference is that such Backdoors do not spread automatically (as Net-Worms do), but only upon a special “command” from the malicious user that controls them.