Botnet is a jargon term for a collection of software robots, or bots, which run autonomously and automatically. They run on remotely controlled groups of "zombie" computers. This can also refer to the network of computers using distributed computing software.

While the term "botnet" can be used to refer to any group of bots, such as IRC bots, the word is generally used to refer to a collection of compromised computers (called zombie computers) running programs, usually referred to as worms, Trojan horses, or backdoors, under a common command and control infrastructure.

http://en.wikipedia.org/wiki/Botnet