I'm not sure on class pre-reqs for the jobs, but I'll tell ya what I know.
PLD: Tank. Need CNJ to 15. Advanced form of GLD. Uses edged weapons and shields. Better at Single target tanking like bosses (IMO).
WAR: Tank. Uses Great Axes. I find them more squishy than PLD, but they have more HP to make up for it. Better at AoE tanking, like trash on the way to bosses (IMO)
WHM: Healer. Advanced form of CNJ. Uses Staves and one handed things. Your sterotypical healer. Has the best form of Protect and Stoneskin.
SCH: Advanced form of ACN. Uses a faerie to help heal and buff. They seem to specialize in wards that absorb dmg rather than just straight healing.
DRG: Damage dealer. Advanced form of lancer. Does not use a wyvern pet. Pretty good DPS if you use your combos effectively.
MNK: Damage dealer. Advanced form of PUG. Specializes in combos by using forms. Dependent on their combos building up. If they are idle too long, their combo fades and they do worse damage.
BLM: Magic Damage dealer. Advanced form of THM. Very good AoE damage dealer. Can tear aggro from the tanks if your blow all your big spells at the start. Gotta know how to ride that aggro threshold.
SMN: Magic damage dealer. Advanced form of SMN. Best DoT job. Weak burst damage, they specialize in DoTs. More effective in longer fights.
BRD: Ranged physical Damage dealer. Advanced from of ARC. Use songs to support party at the cost of own offense. Can do crazy damage if mastered. They are classified as a damage dealer but they use healing Limit break.
The chats are the same as 11. When you get into a free company, to talk, use /fc.
There are no subjobs per se. Classes can learn abilities from the other classes, but they aren't ass effective. Jobs use abilities and traits from their base class (PLD uses GLD skills and traits) and can use select abilities from 2 of it's support class. PLD can use CNJ spells and a couple MRD abilities. Traits are exclusive to the native class. they are not transferable.
Arcanist (ACN) has 2 specs as you can see. They can become both SCH (a healer) and SMN (damage dealer). Note however that when you spend your points, you spend them inyour class, not your job, so if you spend your 20 points in INT, that affects your SCH and your SMN. Both jobs share that bonus so you have to choose which spec you wanna go in. You can also split it between INT and MND. That 20 points doesn't make a huge difference. I spent 20 point in INT and my SCH doesn't seem to suffer for it.