Classes represent chosen professions taken by characters and some other creatures.
Classes give a host of bonuses and allow characters to take actions that they otherwise could not, such as casting spells or changing shape.
As a creature gains levels in a given class, it gains new, more powerful abilities.
Most ¬P Cs gain levels in the core classes or prestige classes, since these are the most powerful.
Most NP Cs gain levels in NPC classes, which are less powerful (see Creating NPCs).