Botnet:

Also called “Zombie Army”.

A network of a large number of private computers infected with malicious software.  They are then controlled as a group, without the owners’ knowledge.  They are usually used to generate spam, relay viruses, or flood a network or Web server with excessive requests to cause it to fail (see “Denial of Service Attack“).

The individual computers are compromised via a Trojan that often works by opening an Internet Relay Chat (IRC) channel that waits for commands from the person/organization in control of the botnet.

There is a thriving botnet business selling lists of compromised computers to hackers and spammers.