What is a bot? A bot is a software robot that automatically connects to command and control servers. The command and control servers then instruct the bots to carry out illicit activity such as performing denial of service attacks, injecting “comment spam” on blogs and forums, or harvesting application content. Building these networks of bots, or botnets, has become a lucrative business tor botnet operators, who rent out their bots to the highest bidder.
Taken from: Imperva: Botnets at the Gate