New Player with Class and Job Questions

[Sagittarian]

And space for items.

Except that at the moment you 1) can't friend your alts/anyone who is not online at the time and 2) can only send stuff to friends.

There are 4 disciplines (Disciple of X) but 19 classes.
Each class has its own xp track. So you can be GLD 30, ACN 17, BTN 3,.... There is no tracking of an overall character level.
Your class is set based on your primary "weapon" equipped - if you are using a sword you are a GLD/PLD, with a needle you are a WVR.
You can save and switch between equipment sets, so you don't have to go through and re-equip everything when changing classes.
Most of the equipment is marked as either GLD/MRD, DoW, or All Classes. There are a few armor pieces that are GLD/MRD/LNC, so lancers are actually the best armored units after the tanks. Class specific equipment mostly shows up at level 50.
There are cross-class skills but the crafting skills are limited to other crafting classes - you can't craft anything as a GLD (while wielding a sword)
The DoH and DoL classes mainly get xp by crafting/gathering. I imagine you could try to kill mobs as one of those, but I haven't. Their listed attack is so weak the only things you'd be able to fight would be so low level as to not be worth it.
Yes, the DoL classes get stealth to make it through higher areas. Those skills are NOT available for cross-class.
Many quests can be turned in as any class, so you can run around and do fetch/carry quests as DoH/L. Also, you can often turn in quests with a different class than the one that "completed" it - go kill X monsters as GLD then turn it in as FSH. Quests will have a minimum level they can be turned in at though, so you can't use this to level lower classes by turning in level 50 quests.
Your 'role' is determined by your class/job - tank, dps, or healer. After level 30 (and some other conditions) you can pick up quests for soul crystals giving you access to the jobs. So far only the Arcanist (ACN) has access to a job with a different role than the base class. Arcanist is dps and can split into either Summoner (dps) or Scholar (healing). Otherwise the job has the same role as the base class but is more suited for party play.
The Duty Finder (DF) is the main/easy way to run a dungeon. It automatically matches you up with people from any server who want to run the dungeon at the same time. The DF has set numbers of each role it looks for in each dungeon, though. For 4 character dungeons it is 1 healer, 1 tank, 2 dps. So your role is extremely important when trying to use the DF.
If you find enough people on your own server (by shouting or through the party finder/PF) to form the needed party size, then you can enter with any combination of roles.