In Darkness Falls there is 8 classes to choose from. You are prompted to choose your class with the items you are given when you first enter the world. Each have their own quest lines and unlockable skills.
Class books can be crafted later in the game progression so other class masteries can be achieved.
Each class comes with a questline. Completing the quest will often give you a few free level in skills for that class or some items. You can find details on quests for each class on it's wiki page.
Each class can be mastered individually. Mastery can be achieved in one of three ways: