What is C2 - Command and Control?

Command and control (C2) is often used by attackers to retain communications with compromised systems within a target network.

They then issue commands and controls to compromised systems (as simple as a timed beacon, or as involved as remote control or data mining). It's usually the compromised system/host that initiates communication from inside a network to a command and control server on the public internet. Establishing a command and control link is often the primary objective of malware.