Straight from the horse's mouth:
https://www.pathofexile.com/forum/view-thread/1596094"Ascendancy Classes:
Nineteen souls survived the Trials of Ascendancy. Nineteen men and women stood at the gates of the Lord's Labyrinth. Nineteen challengers whose exceptional martial and mystical talents still linger long after their death.
Upon completing the Lord's Labyrinth for the first time, your character can choose one of the three Ascendancy Classes available to its core class.
There are nineteen total Ascendancy classes. Each core class has three Ascendancy classes available that embody a different playstyle. The Scion has one Ascendancy class that reinforces her mastery of the passive skill tree.
Upon completing the Lord's Labyrinth for the first time in each difficulty level, you receive two Ascendancy Skill Points you can spend on skills available to your Ascendancy Class.
Respeccing an allocated Ascendancy Skill costs five passive skill refund points. Once you have refunded all your allocated Ascendancy points, you can pick a different Ascendancy Class (at no cost) by completing the Labyrinth again and selecting a new class. Once you have chosen an Ascendancy Class, you can view its skill tree on the Passive Skill Tree near your character's start location."