Close quarters combat or just CQC is the term mostly used by special forces operators. In fact, CQC is a tactical concept that involves a physical confrontation between several combatants. It can take place between military units, police/corrections and criminals, and other similar scenarios. In the last 20 years, a lot of major events permanently changed the world. The warfare shifted from large-scale operations to the small precise raids carried by special forces.
In warfare, CQC usually consists of small specialized units or teams engaging the enemy with small arms at the very short range, up to 100 meters, from proximity hand-to-hand combat to close-quarter target negotiation with short-range firearms.
In the typical close quarters combat scenario, the offenders try a very fast, violent takeover of a vehicle or structure controlled by the defenders, who usually have no easy way to withdraw. Because enemies, hostages/civilians, and fellow operators can be closely intermingled, close quarters combat demands a rapid assault and a precise application of lethal force. The operators need great proficiency with their weapons, and the ability to make split-second decisions in order to minimize accidental casualties.
Much material relating to close quarters combat is written from the perspective of the authorities who must break into the stronghold where the opposing force has barricaded itself. Typical examples would be commando operations behind enemy lines and hostage rescues.
Since the global war on terror, many things changed including the close quarters combat tactics and techniques. Navy SEALs, Delta, Green Berets, and all other SOF units are mostly unitarised their tactics and techniques, but still, they are developing their own styles with a slight difference between each. I have found a rare video of Navy SEAL operators training the CQC. Since then, principles of assault are relatively same: detailed planning, surprise, methods of entry, speed, the violence of action.