Characters get 2 full actions on each of their combat turns. Out of combat, characters can do about 20 actions per minute.
Primarily, full actions can be converted to movement or spent to activate abilities.
Characters get 1 quick action on each of their combat turns.
Primarily, quick actions can be used to step out of range of a reaction, and interact with objects and terrain.
Whenever an opening is provoked against another character, those characters may act out of turn. To do so, they must have actions left over from their previous turn.
Openings are commonly provoked whenever a non-melee action is performed within reach of another character.
Combat actions can be enhanced in one of two ways by spending an ability slot:
Most combat actions gain additional effects when they are amplified. Attack loses its cooldown, stat actions double their targets or accuracy, and abilities all have unique effects.
Quickened actions can be performed with a character's one quick action per turn. This allows much more flexibility with your resources, but actions can't become both quickened and amped.