CQB

CQB is an acronym for Close Quarters Battles and refers to fighting methods within buildings, streets, narrow alleys and other places were visibility and maneuverability are limited.

Both CQB and MOUT are related to urban warfare but while MOUT refers mainly to the macro of it (i.e. sending troops, using of heavy armoured fighting vehicles, battle management), CQB refers to the micro of it - namely: how a small squad of infantry troops should fight in urban environment and\\or inside buildings in order to achieve its goals with minimal casualties.

As a doctrine, the CQB details about topic such as:

Weapons and ammunitions most suitable for the mission.
Extra gear such as bullet-proof vests and night-vision devices.
Accurate explosives.
Routines and drills for engaging the enemy, securing a perimeter, clearing a room etc.
Team maneuvers.
Methods and tactics.

Regulary, only elite military SF units and elite police CT units (such as SWAT) are trained in CQB. Also need to note is that military CQB doctrine is different from police CQB doctrine, mainly because the military usually operates in hostile areas while the police operates within friendly population.

Armies that often engage in urban warfare operation may train most of their infantry in CQB doctrine.