In Architecture, a turret (from Italian: torretta, little tower; Latin: turris, tower) is a small Tower that projects from the wall of a building, such as a medieval Castle or baronial house. A building may have both towers and turrets; turrets might be smaller or higher but the difference is generally considered to be that a turret projects from the edge of the building, rather than continuing to the ground. The size of a turret is therefore limited by technology, since it puts extra stresses into the frame of the house. It would traditionally be supported by a Corbel.
A turret might have a flat top with crenellations as in the picture, a pointed roof, or any other kind of top. It might contain a staircase if it projects higher than the building. However, a turret might not be any higher than the rest of the building; in this case it is part of a room, that can be simply walked into - see the turret of Chateau de Chaumont on this collection of turrets, which also illustrates a turret on a modern skyscraper.
The most common modern use of the term turret is for the rotating cabin installed on a Tank or ship equipped with a large calibre Gun. In modern main battle tanks, the turret is manned by 2 or 3 (the tank-commander, the Gunner and usually the gun loader) and can rotate 360 degrees. It is armed with a large Caliber gun (a Tank gun of 105 mm, 120 mm or 125 mm) and has independent scope systems and outer armour to protect the tank crew.
Rotating automatic cannon or Machine gun post on other vessels such as APCs, naval ships and World War II-era bombers can also be called a turret. Some of those turrets are automatic and controlled from within the vessel and others are manned. Not all of those turrets have armoured protection.
Examples: