Please explain this job thing :)

[Naria]

You should unlock the job for your class as soon as you hit 30. By the time the Main Scenario Quest takes you to Brayflox's Longstop (dungeon) at around 32 you will be expected to have it unlocked. When you unlock a job you receive a job crystal for that job which is equipped in the lowest right slot on your character. A job is basically an overlay on it's base class, giving you new abilities and stats, while still allowing you access to the old ones. For example as a White Mage you would still have all of conjurer's abilities (Cure, Protect, Medica etc) but the crystal will also let to access whm's (Regen, Benediction, Holy etc). Job and classes share exp so when you unlock whm at 30 and equip it to your cnj, you will be a 30 whm.

Heavensward jobs (Astrologian, Dark Knight and Machinist) work a bit differently as they start 30 and have no base class, but you won't need to worry about them until you get to Heavensward.

Edit:
Since you mentioned Scholar, Arcanist is the only class in the game with two jobs for it: Scholar (Healer) and Summoner (DPS). It was an experiment by the dev team for ARR, but they seem to be moving away from it now so it is unlikely that any other class will get a second job.