Botnets

jargon term for a collection of software robots, or bots, which run autonomously and automatically. They run on groups of zombie computers controlled remotely

A botnet operator sends out viruses or worms, infecting ordinary users' computers, whose payload is a malicious application -- the bot.   The bot on the infected PC logs into a particular IRC server (or in some cases a web server). That server is known as the command-and-control server (C&C).   A spammer purchases access to the botnet from the operator.   The spammer sends instructions via the IRC server to the infected PCs, ...   ...causing them to send out spam messages to mail servers.