Update: Since a lot of people seem to come across this page looking for a leveling guide (sorry, mine isn’t finished yet), here’s a link to one that does a decent job at showing you how to reach the level cap quickly with any class or job.
As I said that you can meet every and any class, you can also get any and every job if you have the free time to grind that much. Jobs are basically super classes. They are a combination of two classes and gain special skills but have very little affinities. To unlock a job, you must level its required main class to 30 and its subclass to 15.
The jobs are as follows: level 30, Pugilist and a level 15 Lancer equals a Monk. His affinities are Pugilist, Lancer, and Archer skills. 30 Lancer and a 15 Pugilist will get you Dragoon, and his affinities are Lancer, Puglist, and Archer skills. Level 15 Archer and a 15 Conjurer will get you Bard, as his affinities Archer, Conjurer, and Thaumaturge. A 30 Marauder and a 15 Gladiator will open up Warrior, whose affinities draw from Marauder, Gladiator, and Pugilist. A 30 Gladiator and a 15 Conjurer make a Paladin, who pulls from Gladiator, Conjurer, and Marauder. For the casters, a 30 Conjurer and a 15 Gladiator unlocks White Mage, who pulls from Conjurer, Gladiator, and Pugilist. Black Mage comes from 30 Thaumaturge and 15 Pugilist, and he gets his skills from Thaumaturge, Archer, and Pugilist.
Now the last two jobs are Summoner and Scholar, and they are unknown at the moment, but it can be assumed that they come from Arcanist. Scholar is guessed to be a heal over time style healer while Summoner is thought to be a damage over time caster.