One of the most daunting things for a new gun owner is dealing with the terminology. Below is a list of some of the more common terms and what they mean. This is obviously not all inclusive, and mostly just covers the basics.
Action: The group of moving parts for loading, unloading, and firing a gun.
Automatic: A gun that fires many times rapidly while the trigger is held down. A "machine gun."
Barrel: A metal tube through which the bullet travels to leave the gun.
Bore: The inside of the barrel.
Breech: The back end of the barrel, opposite of muzzle.
Bullet: The projectile, usually made of lead.
Butt: The back end of the stock or grip.
Caliber: System of measurement of the bore of the gun.
Carbine: A short barrel rifle.
Cartridge: Sometimes referred to as a "round", the cartridge contains the primer, case, powder charge, and the bullet.
Case: The metal tube that contains the powder and forms the body of a cartridge. Sometimes referred to as "brass" as this is a common metal used.
Chamber: The part of the gun where the cartridge resides just prior to being fired. In a revolver, the chambers are in the cylinder. In a semi-automatic, the chamber is at the breech of the barrel.
Choke: A slight tapering of the barrel of a shotgun just before the muzzle. This tapering controls how tightly the shot is compressed before it leaves the gun.
Choke tubes: Short pieces of metal which screw into the muzzle of a shotgun that allow you to change the choke of the gun.
Cock: Prepare the gun to fire, typically by pulling back the hammer.
Comb: The top part of the stock of a shotgun.
Cylinder: The part of a revolver where the cartridges are loaded. It spins as the gun is fired.
Cylinder catch: A button or lever which is used to release the cylinder on a revolver so it swings out.
Decocking lever: A lever on some guns used to help safely uncock the gun.
Double action: When the trigger is pulled on a double action gun, it pulls the hammer back before releasing it.
Dry fire: To pull the trigger on an empty gun. Useful for practicing techniques like trigger squeeze, sight alignment, and holding the gun steady while pulling the trigger.
Ejector: Mechanism in the inside of the action which ejects a spent casing from the gun so a new one can be loaded.
Ejector rod: A rod of metal in a revolver used to manually eject cartridges from the cylinder.
Firing pin: A small piece of metal inside the action which strikes the primer of a cartridge causing it to fire.
Frame: The backbone of the gun, the framework to which all the other parts are attached.
Gauge: The measurement of caliber specific to shotguns.
Grip: The part of a gun you hold on to, most commonly used to refer to handguns. Often made of wood, rubber, or plastic.
Hammer: A piece of metal on some guns used to strike the firing pin when the trigger is pulled.
Magazine: Stores ammunition. For many handguns, it is often removable and can be loaded ahead of time for quick reloading. Sometimes incorrectly referred to as a "clip".
Magazine release: A button or lever on a gun which is used to release the magazine from the gun.
Muzzle: The end of the barrel where the projective exits, the "business end".
Powder: A chemical compound (gunpowder) that catches on fire and burns rapidly. This produces a lot of gas which expells the bullet out of the gun.
Primer: The part of a cartridge that is struck by the firing pin to make a spark which ignites the powder charge.
Receiver: The outside frame of the action where information about the gun (caliber, chamber length, serial number, etc.) is often stamped.
Safety: A mechanical device in the action of a gun designed to prevent the gun from firing if the trigger is pulled. As a mechanical device, it can break so should not be relied upon totally.
Scope: An optical sight similar to a small telescope used most commonly on rifles but also occasionally on handguns and shotguns.
Semi-automatic: A gun that will fire each time the trigger is pulled and reloads a new cartridge each time. Not to be confused with "automatic".
Single action: The gun must be cocked before each trigger pull.
Sight: Used to aim a gun, typically a combination of a front sight and rear sight which must be aligned.
Slide: The outer portion of the action on a semi-automatic handgun which slides back and forth as the gun fires.
Snap cap: A "dummy round" that is used to dry fire a gun. It contains a spring inside to absorb the energy of the firing pin.
Stock: Rear and front outer portions of a rifle or shotgun used for holding the gun. Typically made of wood, but also can be plastic.
Trigger: A lever on the lower portion of a gun used to fire the gun.
Trigger guard: The semi-circular piece of metal around the trigger which helps prevent it from accidentally being pulled.