Botnet
Alternatively referred to as a zombie network, a botnet, short for bot network, is a group of infected computers that are under the control of one or more individuals. The infected computers are used to perform complex tasks that would be hard for one computer or a task someone would not want to be caught doing, such as distributing SPAM e-mail or a Distributed Denial of Service (DDoS) attack on other computers or networks.
Also see: Bots, Security definitions, Zombie