It's not class based; you choose freely from a multitude of skills (that are labeled under different classes purely cosmetically). Contrary to what you might think, you don't have to learn all the skills before a skill before you can put points into it; all that matters is that you fulfill the level requirement (and have points to spend, duh ...).
Later on you will be able to relocate your points if you wish, so don't worry too much if you regret your choices. It costs money, though. --- I could be bounded in a nutshell and count myself a king of infinite space -- were it not that I have bad dreams ...
There is a post somewhere about the story of Divinity - BUT if you know too much it does give away a little of the story of this one inevitably :) You don't really need to know any more than the story itself tells you.
2: the game itself , gameplay
Best to check an FAQ for this one - but take your time and explore is essential - don't rush to the major quest goals.
3: Or any class recommendations ?
There aren't classes as such, but you will want to specialise your skills. Magic is probably easiest to specialise in. By specialising, you create your own class from the skills available. Don't spread your points wide - focus in on 3-4 skills in total!
Level up (and increase via skill trainers) your wisdom, lockpicking and encuberance...these three are the most important.
Also, get high levels in explosive arrows, summon demon, healing, duel wield and fireball.
The demon is a handy assistant when fighting nasty enemies who keep healing themselves again and again....gives you time to shoot them with a explosive arrow. --- "If Ripto were here, I'd give him a piece of my mind!" : Moneybags from Spyro 2