You have to first reach level 10 on your current class to unlock the Armoury system which gives you the ability to freely change classes by changing your weapon.

Example:
Equip a bow ---> Archer
Equip a lance ---> Lancer
Equip a Needle ---> Weaver
Equip a Pickaxe ---> Miner

You can swap anytime out of combat once you have access to the Armoury system. Each class is individually leveled up.

There are varying tiers:

-Discipline: broad term to describe what type of class you are. Disciple of Land = gatherer, Disciple of Magic = magic user, etc.

-Class: Archer, Gladiator, Conjurer, etc based on your currently equipped weapon

-Job: earn a Soul Crystal at level 30 on your class to specialize your class and "upgrade" it to the appropriate job. Thaumaturge becomes Black Mage, Archer becomes Bard, etc.