A job can be considered essentially a specialization of the class. A White Mage, for instance, is still a Conjurer, and the experience you earn as White Mage will advance your Conjurer class and unlock Conjurer skills. (This is why it's not helpful to max out CNJ first, because you'll end up doing that anyway while playing WHM.) But your job quests will also unlock additional skills that are only available with the job. So while you technically could advance CNJ to 50 or 60 by playing either CNJ or WHM, you're better off doing it as WHM. The same goes for any of the other Realm Reborn jobs and their associated classes. (As Naria mentioned, the new jobs introduced with Heavensward work a little differently, and don't have base classes. They're more like a combination of class and job rolled into one.)