Aka, a Conjurer can specialize into a Black Mage or White Mage.
I imagine the jobs are going to have advanced abilities specialized for party play while the classes will maintain the diversity for solo or small groups. Jobs will require a quest to unlock and a class rank requirement.
Bayohne has posted on the Lodestone forums:
Spoke to the dev. team to try and wrangle some more details regarding the upcoming job system. The current thinking is to have one job for one class. So for example, if you are a gladiator, the job you could unlock would be paladin, etc. Right now, we can't give any further details about the possibility of having multiple jobs per class.
I realize I'm kind of nitpicking, but I just wanted to give the most accurate information possible. I apologize if this seems rude. ;_;