Jobs are for job use at times when jobs can be used. Classes are for anytime anywhere. Jobs have to quest for their action and abilities. Jobs have to quest for gear that is specific this includes weapons. Job actions and abilities are not interchangable with other jobs, they are specific.
You have to have the matching class/classes to the correct level to be able to quest for the job. He did say in one interview it would be something like Gladiator to lvl 30 and marauder to lvl 15 as a "unconfirmed example" to get Paladin.
This is from the interviews I have read.
Main thing to note is that Jobs are not going to replace Classes, but are for use only when allowed eg: parties (maybe) and quests (maybe), instanced/raids (again maybe).
The thing is nothing is set in stone yet, he said, so we will find out how it all really will work when he tells us, until then why worry about it, we have a fair while yet.