Spirit guides allow you to undo your skill point selections on the skill tree, allowing you to to reallocate them. You can only reduce your mastery to 1, therefore you cannot choose a new class through this method. In addition, you cannot remove the last point from a skill that has any modifiers with points invested in them.
The cost goes up every 20 points refunded. The initial cost is 25, then 50, 100, 150, 200, 300, 400, 500, (Not sure after this).
Spirit guides also allow the refunding of Devotion points.