Ascendancy Classes The Help Panel
Although any class in Path of Exile can eventually reach any passive skill on the passive skill tree, each class has their own distinct set of Ascendancy classes. A Witch can, for example, eventually become an Elementalist, but a Templar cannot. You can unlock your Ascendancy class by completing the Labyrinth and using the Altar of Ascendancy in the final room. Higher-tier Labyrinths then grant two more Ascendancy points when you reach the Altar of Ascendancy, for a total of 8 points.
Once you've chosen your Ascendancy, you can change it by returning to the Labyrinth, refunding any allocated Ascendancy points (Refunding an Ascendancy point costs five refund points each), and selecting a new class at the Altar of Ascendancy.