Action point
What heroes spend during battle to use actions. Normally, a hero can use two action points per turn, though some abilities can grant additional AP when their conditions are met.
Movement also costs action points.
Free action
A free action is an action that never costs an action point to use, such as Swap Weapon or Volley of Arrows. There is no limit to the number of free actions a hero can perform during their turn.
Swift action
A 
swift action is an action that may not cost an action point to use. Specifically, a hero can use one swift action for free per turn. Subsequent use of the same or any other swift action on the same turn becomes a 
single action.
Single action
A single action is an action that costs one action point to use.
Single action, ends turn
Typically, a 
single action, ends turn is an action that costs one action point to use, and ends that hero's turn. Any unused action points are not carried over to the next turn.
More precisely, single action, ends turn actions consume two action points. This distinction is relevant only in certain special cases.
