Note: These are my definitions - I have tried my best to word it as I would when explaining it to a friend, face-to-face. This is intended to help define words I will be using throughout my site. This is not an exhaustive, conclusive list.

Action: The part of the firearm where most of the other parts (barrel, bolt, receiver) come together. The action is where "the action" is...where the explosion takes place after the firing pin impacts the primer.

Ammunition: Referring to the whole cartridge (case, primer, powder, bullet) that is placed in the chamber and ignites when impacted with the firing pin to propel the bullet out of the barrel. 

Barrel: The cylindrical extension through which the bullet travels to exit the gun. Typically made of an alloy or stainless steel, and for most gun types the barrel is rifled to improve accuracy.

Barrel Twist (a.k.a. Twist Rate): The number of inches of axial ("down range") distance for the rifling to make one complete revolution in the barrel. This varies based on the ballistics of a given type of ammunition, for example a .30-06 Sprg caliber M1 Garand barrel has a 1-10 (1 revolution over 10 inches) twist but a .45 ACP caliber 1911 barrel has a 1-16 (1 revolution over 16 inches) twist.

Bullet: The projectile that gets shot from a firearm. It is typically a malleable metal like lead, and is the "head" part of a cartridge. The bullet goes in first when orienting a cartridge in the chamber.

Caliber: This generally refers to the size of the bullet, but can sometimes vary by cartridge or case diameter. Calibers are hard to think systematically about. For example, a .308 Win bullet and the 7.62x51mm caliber cartridge both measure .308 inches in diameter but are not the same caliber.

Cartridge: A single piece of ammunition, containing the case, primer, powder, and bullet.

Chamber: The part of the barrel where the cartridge is placed. The explosion that sets off a firearm occurs in the chamber. Chambers are typically made to SAAMI specifications.

Cylinder: The cylindrical-shaped part of a revolver that holds the ammunition and contains the explosion when the firing pin ignites the primer.

Firearm: A weapon or tool that contains a small explosion from a round of ammunition, that sends a projectile (bullet or shot) out of the barrel bore. An air rifle is not a firearm, since there is no combustion process to fire it.

Racking: To "rack the slide" is to pull the slide (see below) back and let it go forward into battery. This is often done to load a round from the magazine into the chamber or to eject a round from the chamber (i.e. after a malfunction).

Slide: The metal component of a pistol that reciprocates after a shot is fired. The slide is usually made of steel and covers the barrel and action. It can be gripped and "racked" to load the next round, to lock the slide back, or address a malfunction.