This is just a standard FF kinda thing. I refer you to http://finalfantasy.wikia.com/wiki/Job_System.

I suppose there's nothing wrong with them being called Classes, and indeed they have been in other games. We already had Classes in this game though, and besides, "Job" sounds cooler to me because it speaks specifically to FF in my mind, whereas most MMOs use "Class". Besides, in the first FF MMO they called them Jobs after all, so it makes sense to do it here too imho.