Adventurers are extraordinary people, driven by a thirst for excitement into a life that others would never dare lead. They are heroes, compelled to explore the dark places of the world and take on the challenges that lesser women and men can't stand against.
Class is the primary definition of what adventurers can do. It's more than a profession; it's a calling. Class shapes the way adventurers think about the world and interact with it and their relationship with other people and powers in the multiverse. A fighter, for example, might view the world in pragmatic terms of strategy and maneuvering, and see herself as just a pawn in a much larger game. A cleric, by contrast, might see himself as a willing servant in a god's unfolding plan or a conflict brewing among various deities. While the fighter has contacts in a mercenary company or army, the cleric might know a number of priests, paladins, and devotees who share his faith.
Adventurers sometimes advance in more than one class. A rogue might switch direction in life and swear the oath of a paladin, A barbarian might discover latent magical ability and dabble in the sorcerer class while continuing to advance as a barbarian. Elves are known to combine martial mastery with magical training and advance as fighters and wizards simultaneously.