Yes your character is tied to one server that is shared with PC, PS3, and PS4 players. Also in Duty Finder instances, you group up with people from other servers.

Dragoon is a polarm-wielding job that specializes in jumping and piercing style of combat. It is a melee damage dealing class specializing in single target dps. Dragoons have all the abilities and traits of Lancers. They can also use all the cross-class abilities of marauder and pugilist. The most important stat for the dragoon is strength, which increases attack power. In order to become a Dragoon, characters must have a level 30 Lancer and a level 15 Marauder

Black mages are able to cast devastating spells of magic on multiple enemies. A Black Mage's most important stat is intelligence, which increases their attack magic potency. In order to become a Black Mage, characters must have a level 30 Thaumaturge and a level 15 Archer. The Black Mage can access cross-class abilities from the Archer and Arcanist classes.

Getting your first class or job to 50(Max level) can be done with the main story mission alone (main story is still continuing with patches), after that its not so easy, but you still have a lot of options to choose from.

All the instances requires you to party up. There is 4 player dungeons, or 8 player dungeon like the Bahamut Coil, and boss battles, and the Crystal Tower which is a 24 player raid.

It is not too difficult to level up. The devs made it so that even people new to Final Fantasy and MMO in general can pick up and play at their own pace.

The most difficult dungeon is Bahamut Coil, and the toughest Boss battles are the Primal Extreme. Mostly Titan Extreme.

I was thinking of making a dragoon but I hear they're useless. Is that true?

What outlawauron said is true, and to add to that, with just one character you can level everything,

so having all these options to help with leveling is great if you're a casual player or hardcore imo.

No, it's not true. They're the tankiest DPS class, and one of the more difficult to use. People think that MNK and DRG are worse than the magic DPS, but that's because most people just have no idea how to play their class. 

I don't think so. They have some of the best looking armors in my opinion. They're hard single target hitters, and has the best critical hit rate I think. If you don't like Dragoon you can always unlock another job or class till you find one that suits you best.

As a matter of fact, DPS roles is very numerous across all servers. We need tanks like Paladin and Warrior. Some people feel pressured taking the tanking role because tanks kind of control the pace of a dungeon raid, but I find it extremely fun.

Do PS3, PS4 and pc players play together? Are there any differences besides looks between PS3 and PS4? Thinking about buying this game on PS3.

Yes, PC, PS3, and PS4 players play together. The resolution, draw distance, texture, and frama rate is better on PS4 than on PS3. Mouse and keyboard support on PS4. Faster loading time. It just feels so much nicer on PS4 imo.

Dragoon is the more versitile compared to a Monk when it comes to pure combat and has higher burst and high sustained damage whereas Monks excel at single target dps fights where the boss has a higher stable up time, but like any MMO, ranged will always be easier, espeically if you are a Bard since you can attack on the move at ranged which no other class can, but comes at the cost of lower dps while having the best support abilities compared to a Black Mage or a Summoner.

You will always want a Bard in a raid, and most times a Black Mage, but not always the others is what it comes down to if you really want to get into it, for the songs and big AoE nukes. Melees are your high single target dps guys, and Summoners are very good for certain fights as well but not always needed unless you want that Miasma slow for Coil T7.

I played the beta, and I can't, for the life of me, find anything enjoyable out of the game. It's boring, ugly, and just not what I want final fantasy to be..... Ever.