Each account starts off with three characters, Elesis, Lire, and Arme(In foreign versions, you get a few extra characters, but three for the NA version). As you level up, you're able to buy and complete missions to 'unlock' the other characters, who then start off at level 0 and you're able to level them like any of the other ones.
There are a total of 10 characters out in the NA version, and the level cap is 75.
Class changes start at certain levels(for NA, 2nd is at 20, 3rd at 30 and 4th at 40). When you unlock these, you're given new weapons that you're able to wield and use that job. You can switch weapons and jobs at any given time, but you need to unlock them first.