So which one should you pick? Well, here's what I've gathered about each of them so far:

The Assassin is your regular damage rogue. Contrary to popular beliefs, he cannot stunlock and has in fact very few crowd control abilities.

The Ranger uses bows or crossbows as you might have suspected to range-attack unaware foes. He is weak in melee combat however. What's interesting is that as a ranger, you have to aim manually at your opponents, and mashing a key on your keyboard isn't an option.

The Barbarian lacks damage but has increased crowd control; with many stuns and knockbacks available, he actually is the stunlock rogue. He also benefits from being in a party or raid group, as his damage increases if more people fight alongside with him.

The Priest of Mitra is your (not so regular) healing priest. It's not a static fight for him, and most healing abilities come with a damage component, or are cone ranged.

The Bear Shamman combines healing with damage while shapeshifted.

The Tempest of Set is your damage-flavored priest. Using lightning bolts as his primary attack combined with a good range rooting ability makes him a fierce opponent.

The Guardian is your main tank, although he can inflict heavy damage as well in some cases as he is the only one specialized in using polearms.

The Dark Templar is an unholy warrior, who has tanking, damage and healing abilities, but does not excel at any. He can steal life from his enemies to heal his own party and himself.

The Conqueror is a warrior specialized in dealing damage, whether he does it with a big two hander or two swords, or anything similar really.

The Necromancer is specialized in dealing ranged magic damage and summoning minions to aid him in various situations.

The Herald of Xotli is the battlemage. He uses a two hander, and combines melee damage with magic damage (usually fire). He can make quick work of most enemies, but he wears cloth armor and he is really weak to attacks. Furthermore, he needs to get into melee range to deal damage. He can transform into the demonic Avatar of Xotli for a short period of time, so you should stay out of his way when he does.

The Demonologist is your actual mage, dealing range magic damage. He has only one pet, as opposed to the Necromancer who can summon many.

Let's see how the combat system works

What Funcom calls "Real Combat" is an interesting approach to the standard RPG combat system. Indeed, you have all the abilities and standard attacks of a RPG, but each melee ability (does not apply to spells and ranged) has to be performed as a combo. You start with 3 basic weapon swings: left, front and right. Pushing these buttons (1,2,3 default) will attack your opponent in the respective direction. Each opponent (monster or real player) as some kind of a shield around him. By default, the shield has the same strength on all sides. But you or the AI can choose to weaken one side and strengthen another. You can for example expose your right side and reinforce the left side if the enemy prefers attacking there. You will deal/receive damage all the time, but the amount varies proportionally with this shield and the side you are attacking, taking a hit.

As your character grows, more weapon swing directions are added to your attacks. Special abilities rely on performing a combination of directional attacks in order to be unleashed. -> ,<-, -> for example can trigger an ability. But if you fail to perform the combo (by pressing something wrong), you won't be able to do that special attack.

Spells don't use the combo system, and ranged has its own level of difficulty by having to aim manually.