Defined: Double action, Single action

DA / SA or DAO / SAO
Ever see these designations in an ad for a firearm? This isn't gibberish. Those letters stand for something, and they can be the difference between weather you can pull a trigger, or not.  

Single Action : The trigger only does one thing, release the hammer to fire the gun. A gun designated as Single Action Only will always need to have it's hammer cocked, before the trigger is pulled. If not cocked, the gun will not fire.

Double Action : The Trigger must pull the hammer back AND release it on the firing pin. This trigger will be heavy and hard to pull. If the gun is Double Action Only, you will not be able to first pull a hammer back, in order to lessen the weight of the pull. 

Some pistols that have a standard type of trigger are...
1911 : SAO
Hammerless Revolver : DAO