You can level any job you want. Equip the weapon for any class and have at it. Basically the way is works is you level a class to 45 (some higher) then you can take on a few missions to unlock the main Job.
So Conjurer --> White Mage and so on.
I think that is one of the great things about this game. Unlike most mmo that ties you into your class you have alot of options class wise in this game.
Clarification, its lvl 30
since you gain new quests for new job unlocks every 5 levels. (Like getting regen at lvl 35 for WHM, then a new ability at lvl 40 specific to WHM etc.)
Yes classes are just like FFXI jobs except you dont need to find a moogle to change, you just equip the right tool or weapon and you become the class based on it. Each class has its own levels, and each combat class (Mage or Melee) has at least one job, and this recent producer letter alludes to the addition of a second job for each class. Jobs are basically specs for classes that limit what abilities you can take from other classes, but adds brand new ones exclusive to that job that is specifically designed around a role in a party.
Change Conjuror to WHM, become the ultimate party healer, change Archer to Bard, become a range support/buffer with some damage capabilities, Thaumaturge to Black Mage, become a wizard of elemental destruction.
But if say you want to equip the spell raise to your Thaumaturge from the conjuror ability list, you can't be speced to the Black Mage job so there are times when you might not want to go to a specific job, like for instance low manning instances or content where a more balanced approach is necessary.
They made it sound like that Arcanist, Summoner and Scholar wouldn't be playable in the beta at all, so... I kind of doubt that, unfortunately.
Yoshida mentioned in the previous live letter that we will get new trailers designed specifically for the new class/jobs as well as Coerthas and Mor Dhona zones in the near future, some of those could appear at E3. They just won't be testing those things in Beta because they are high end content and the beta test has a lvl 35 cap for content. They also likely want to keep Arcanist something new for release. Thankfully we just got information the Chocobo buddy system is getting tested in phase 3 now so we can test the pet system for Arcanist
(They also said you can have chocobo buddy AND carbuncle out at the same time for Arcanist at release!)