Command and Control server
A Command and Control, or C&C, server is a computer used to coordinate the actions of computers infected by a bot, rootkit, worm or other forms of malicious software (malware) that rely on another computer for instructions and updates. A C&C server may tell infected computers what sorts of information to steal, provide targeting information for infected computers to attack or infected, update existing or install new malware on infected computers and so forth. A C&C server may (or may not) also serve as a drop zone for infected computers to upload stolen information.
A C&C server does not have to run the same operating system as the computers it controls. Often, a C&C server will be run on a web server that has itself been compromised or has been purchased using stolen credit cards.