A firearm action is the mechanism by which it is made ready to fire. This can be an entirely manual process, or automatic.
Types of Action
There are a wide variety of actions available.
These actions can only fire one shot, before having to be manually reloaded.
A break-action splits on a hinge directly behind the chamber, allowing a cartridge to be loaded.
These actions can fire multiple shots between reloads.
Bolt-action uses a hand-operated bolt to eject a spent cartridge and chamber a new round.
A lever-action uses a lever, which is usually an integrated part of the trigger guard and handle, to manually eject and chamber cartridges.
A pump-action uses a movable fore-stock to manually eject and chamber cartridges.
Revolvers have multiple chambers which are rotated into place for firing.
A semi-automatic uses energy from a fired round to eject and chamber cartridges with each trigger pull.
A fully-automatic uses energy from a fired round to eject and chamber cartridges continuously.
Single-action triggers only fire, they require a separate action to cock the striker or hammer.
Double action trigger mechanisms are capable of both cocking the hammer or striker and firing the weapon.
Double-action/single-action trigger system are capable of cocking the weapon and firing, but only the first trigger pull cocks the weapon. All subsequent cocking actions are performed by some other aspect of the firearm's action.
Double-action-only trigger mechanism rely on the trigger pull to cock and fire every single time, this often considered safer and yields a more consistent trigger pull.