Chamber (firearms)

The chamber is that part of a firearm into which the cartridge is loaded. The chamber is also the place the cartridge is fired from. In a breech-loading weapon the chamber is directly behind and in line with the barrel. In a revolver there are multiple chambers contained in a revolving cylinder. 
The typical number is 6, which is the reason for the nickname Six Shooter and indicates the cylinder has six chambers each holding one cartridge. Most rifles and pistols have a single chamber. To "chamber" a round means to load a cartridge into the chamber.
Semi-automatic weapons.
In a semi-automatic firearm, the weapon can fire a bullet each time the trigger is pulled (until the magazine is empty). A semi-automatic firearm can be a pistol, rifle, or shotgun. Each time the trigger is pulled, the gun fires, the spent shell is ejected and a new round is loaded into the chamber. The semi-automatic is a very popular choice for sporting use.
Semi-automatic pistols and rifles use the pressure of the fired cartridge to move the slide backwards. A fraction of a second later, the slide moves forward loading a fresh cartridge into the chamber. The magazine uses a spring to feed one cartridge at a time into the chamber.
Fully automatic weapons.
Fully-automatic firearms are those that automatically reload and fire again repeatedly as long as their triggers are depressed and they still have ammunition in their magazines. Fully automatic weapons are not for sporting use but are used by police and militaries. An assault rifle is "a gun that can shoot many bullets quickly and that is designed for use by the military". Like semiautomatic firearms, automatics have a single chamber.
Manual loading weapons.
A manually loaded firearm is one in which the chamber is loaded by hand each time the weapon is fired. For example, a Bolt action rifle ejects the spent cartridge when the bolt is pulled back, then loads another cartridge into the chamber when the bolt pushed forward (and locked). 
A lever action rifle does the same thing when the lever is moved down (ejects the spent cartridge), then up (loads a new round in the chamber). There are also single shot pistols, rifles and shotguns in which the breech is opened to eject a spent shell, then the shooter loads a round into the chamber manually. 
A popular type of manually loaded firearms for sporting and hunting use is the pump action shotgun. Here, a sliding handle (usually located under the barrel) is moved backwards to eject the spent shotgun shell, then when moved forward it loads a fresh shotgun shell from a tubular magazine.