From what I understand is jobs are for the casual player (War), and classes are more for the solo hardcore player (MRD). Again this is how I understand it and I could be wrong but this is how I believe the system so far works.

Jobs (like warrior or monk ex..) will have set abilities/skills/moves, a balance for that job at which you can not use skills from other jobs, your just a job with a set of skills. Classes (like MRD or PUG Ex..) will continue to have their own set of abilities/skills/moves, how ever, you will be able to use skills from other classes kinda like you can do now, example would be using cure from a conj on your MRD ex...

Easiest example would be GLD VS the PLD: A PLD (job) will have cure, a taunt type move and so on, so if your a casual player and like tanking this is how you would achieve that goal just rank up a gld which will turn into a pld and thats it, changing to PLD will have your basic skills as a tank that's all you have to level up. Now lets say your a hardcore player and want a tanking job, than again you would rank up a GLD, but now you have to level up a conj as well for the cure, and a pug for the taunt and so on, basically making you rank up other classes (kinda like todays system) and using the skills from other classes on your GLD.

Hope this helps, and again this is how I believe the new system works, I could be wrong because I'm getting bits of information from here and there, but that's pretty much how I think this will work.