Published by Silicon.com Aug07
A bot, short for robot, is an automated software program that can execute certain commands. A botnet, short for robot network, is an aggregation of computers compromised by bots that are connected to a central "controller." These botnet controllers are often controlled from chat rooms. Botnet controllers can be linked together to form even larger botnets and to replace another controller that is taken off line. Botnets controlling tens of thousands of compromised hosts are not uncommon. Botnets are becoming a significant problem across the Internet.
A white paper from US-CERT is available here http://www.us-cert.gov/reading_room/malware-threats-mitigation.pdf