Botnet is a jargon term for a collection of software robots, or bots, which run autonomously and automatically. They run on groups of "zombie" computers controlled remotely by crackers. 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. A botnet’s originator (aka "bot herder") can control the group remotely, usually through a means such as IRC, and usually for nefarious purposes
Got from : http://en.wikipedia.org/wiki/Botnet
Botnets are group of PCs infected with malware designed to steal information. They are web robots.