You can level every class, but you can only be one class at any given time, which is determined by your main hand equipment. If you're following the main story quests and your class quests, which you should be, between levels 10 and 15 you'll be told you're allowed to change classes. At that point, you can go to any other class's guild, take the quest from the receptionist, and unlock the class. Each class levels separately, but you can share common gear between them.

This means that you can be a warrior and a crafter, but not both at the same time. You either wear your warrior gear and fight or your crafting gear and make stuff.

As for jobs, they just sit on top of classes via a specific equipment slot.