In the Armoury System, changing your adventuring style—or class—is as simple as changing what you hold in your hand.
Each class is capable of performing unique actions which are learned as you gain experience and your level rises. Furthermore, achieving a certain level of mastery in multiple classes will unlock jobs, which feature even greater specialization.
Classes are suited to those who desire versatility, as they allow the use of actions that belong to other disciplines. In contrast, jobs grant access to exclusive actions and are ideal for situations that demand well-defined roles. Which fighting style you adopt is entirely up to you.