From BS manual:

In the combat system of The Banner Saga, regular actions are Movement, Attack (to Armor or Strength) and Rest. Abilities are special actions that can be performed or take place during a Battle that transcend regular actions. Abilities can be:

• Passive, events triggering without the unit having to initiate them, so in a sense they are not actions.
• Active, actions replacing a regular Attack and depleting 1-3 Willpower points to execute.

See:

• https://bannersaga.gamepedia.com/Abilities
• http://xcom.wikia.com/wiki/Category:Class_abilities_(XCOM:_Enemy_Unknown)