DDoSPedia is a glossary that focuses on network and application security terms with many DDoS-related definitions. It provides a central place for hard to find web-scattered definitions on this topic.
A botnet is a collection of compromised computers often referred to as "zombies" infected with malware that allows an attacker to control them. Botnet owners or "herders" are able to control the machines in their botnet by means of a covert channel such as IRC (Internet Relay Chat), issuing commands to perform malicious activities such as distributed denial-of-service (DDoS) attacks, the sending of spam mail, and information theft.
As of 2006, the average size of any given botnet around the world was around 20,000 machines (as botnet owners attempted to scale down their networks to avoid detection), although some larger more advanced botnets such as BredoLab, Conficker, TDL-4, and Zeus have been estimated to contain millions of machines.