Ability points
Spending ability points
You can 1 ability point per level, which can be invested into ability enhancements.
You can open your spell book via its icon within the left-panel or by pressing the k shortcut (while the textbox isn't in focus).
The following illustrates the Rogue's Embrace the Shadow ability and the enhancement that is available that allows you to reduce its cooldown:
As you get higher level and earn more abilities, there will be more enhancement options than ability points available -- so choose wisely!