Here's what info we have on jobs and my thoughts on what each means.
Quotes are from Matsui's letter except where noted.

Quests to unlock each job will become available once you have reached a certain rank in the corresponding class.
This points to jobs being directly related to a class, whether this will mean jobs can only use the weapons of that class or not, well, we will have to wait and see, and job specific gear mentioned below may allow for that.

Rather than being advanced versions of classes, we would like players to think of jobs as specialized playing styles optimized for partying. While playing as a particular job will render you unable to equip certain actions of other classes, you will be compensated with access to exclusive actions, which can be acquired through quests.
This one can be pretty confusing, but basically is saying that it will focus on your class instead of a jack of all trades master of none.

In practice, players will find the existing classes effective when playing solo or in certain party setups. Jobs, however, will shine in party situations where specific skill sets are demanded.
So jobs are designed for parties, and from this following quote.
Quote Originally Posted by Letter from the Producer X
Shift to automated character development based on class rank, with optional attribute point allotment
Class seems to be the core of the leveling system, so jobs would be like equipping an item and not a separate class system. (not saying that you'll have to put on a piece of gear to switch to a job)

Now, you didn’t hear this from me, but there are also rumors circulating that quests for obtaining job-exclusive equipment are also in the pipes.
Gear that further focuses on the role of a job. Sounds good for those regularly party or do end game content. (I know, not much of that yet unless you like killing the same 5 NM over and over ha ha.


To summarize
Job rank = class rank
Different weapons = unknown
Job focuses on current class and job abilities
Jobs are for party situations
More gear to collect