The Witch Doctor, abbreviated WD, is a pure spellcaster, dealing with elemental magic, nature magic, necromancy and alchemy. The class has taken some inspiration from the Necromancer of Diablo II.
The Witch Doctor comes from the an area of the Torajan jungles called Teganze on the southern tip of the great eastern continent, just southwest of Kurast (where Act Three took place in Diablo II). You can see it located on the World map close to Kurast.
The fearsome Witch Doctor of the umbaru is believed to be a legend by many, but Abd al-Hazir has seen them with his own eyes, documented in his thirteenth entry of his Writings.
The Witch Doctor uses terrifying precision to assault his foe's mind and body with elixirs and powders evoking fires, explosions, and poisonous spirits. He can also summon and control undead creatures from the netherworld to combat his enemies.
He and she comes from the Tribe of the Five Hills, who frequently engages in tribal warfare with both the Clan of the Seven Stones and the Tribe of the Clouded Valley as a ritual rather than conquest. The tribes use these wars to replenish their supply of human sacrifices their civilization revolves around. Only those taken in battle are considered worthy of the ritual sacrifice according to the code of heroism and honour of the tribes.
The tribes define themselves by their belief in the Mbwiru Eikura, which roughly translates to "The Unformed Land". Their belief holds that the true, sacred reality is veiled behind the physical one we normally experience. Their vitally important public ceremonies are centred upon sacrifices to the life force that flows from their gods, who inhabit the Unformed Land, into this lesser physical realm.
Witch Doctors are finely attuned to this Unformed Land and are able to train their minds to perceive it through a combination of rituals and the use of selected roots and herbs found in the jungles. They call the state in which they interact with this other world the Ghost Trance.
The second most sacred belief of the tribes is their philosophy of self-sacrifice and non-individuality, of suppressing one's self-interest for the good of the tribe.
Their most current war created an intense social upheaval among the tribes due to an incident, of which details are lacking, but might be the reason the Witch Doctor has travelled west to Tristram.
The Witch Doctor looks a bit like the top half of one of Diablo II's Flayer Shaman, especially when outfitted in one of his oversized, colourful tribal masks. There is even a Flayer Shaman called Witch Doctor Endugu. Judging by the skills thus far demonstrated, the WD fights a lot like a Necromancer, using cunning and guile, rather than direct physical attacks. Witch Doctors possess a number of mind control spells, the ability to summon and control undead servants, and a variety of direct damage elemental attacks.
and fire-enhanced Mongrel, primary minion of the WD.
The D3 Team has described the Witch Doctor as old and grizzled and fanatical. His magic is not clean and easy, like the fire or lightning a Sorceress can produce with a snap of her fingers. Witch Doctors get dirty. They summon walls of zombies that rip apart attacking enemies, they call forth undead demon hounds called Mongrels that they can detonate to deal heavy damage to their enemies, and they fashion the skulls of the dead into explosive grenades. The D3 team discussed the Witch Doctor's design concepts in the Denizens of Diablo panel at the WWI 2008.
- For the Witch Doctor, we wanted to get a rich, voodoo vibe. We wanted to avoid straight up master-caster feel. He's not just conjuring things out of thin air. WDs don't just channel magic. They take real world objects and infuse them with voodoo magic. Our spell descriptions for the WD were like, "He's going to throw a shrunken head filled with a chemical concoction." Or "He's going to create insect swarms by blowing voodoo dust from his palms." Or "He's going to sprinkle dust in the air and cause illusions and frighten monsters away."
The team felt that it wouldn't be a real Witch Doctor without zombies. They go together naturally, but they didn't want normal slow wandering zombies. They mashed the idea of Zombies with the Firewall of previous games and came up with zombie wall.
Leonard Boyarsky describes the Witch Doctor: 
- The Witch Doctor is angry, someone who's been broken by a life that's dealt a few too many hard knocks and not enough joy. This is someone tired of being smacked in the head, so he uses his mystical powers to get into the heads of others (and if that doesn't work, a swarm of locusts will get under their skin).
The WD is clearly Necromancer-like, in his mixture of summonings, mind control curses, and sub-mage quality magical attacks, but the D3 team does not view the character as a replacement for the Necromancer.
- Q: Is the witch doctor a replacement or spiritual successor to the Necro? They seem to have rather similar skills.
- A: We don't view the Witch Doctor as a replacement. The Necromancer is a very cool char. We thought about him as a class and tried to see if we could improve on him, which is something we did with every class, and that led us to create the Witch Doctor. If we ever decided to make a Necromancer, the Witch Doctor wouldn't prevent us from doing so.
Jay Wilson commented further on this and detailed what makes the Witch Doctor different than the Necromancer in an interview with 1up.com in December 2008. 
- We wanted to create a class whose pets were not his primary source of damage output. [...] Most Necromancer builds are very pet heavy. The pets do a lot of the damage, and a lot of mechanics are built around debuffing the enemy so your pets can be better against them or taking advantage of the bodies your pets create by blowing them up with corpse explosion.
- The Witch Doctor's pets are more of a distraction -- they're his form of crowd control. [...] We wanted to have this general notion of a character who controlled all things slimy and gross, like zombies, bats, snakes, and spiders, but he didn't rely on them -- he just throws them out there. [...] Each element is like that, where it's another distraction while the source of primary damage is the Witch Doctor himself. This makes him play very differently than the Necromancer, which was intentional.
One of the key changes to spell casters in Diablo III is the addition of +% spell damage modifiers. These sorts of modifiers were found on many skills in Diablo 2, and on a few uniques and runewords, but they were uncommon, and not necessary to kill effectively. As a result, casters in Diablo 2 were much less item-dependent than the combat classes, and found it easier to concentrate on Magic Find, resistances, hit points, and other non-damage bonuses.
This will not be the case in Diablo 3, where mages must add considerable +% spell damage from modifiers to stay ahead of the monster hit point curve. A high level Wizard or Witch Doctor in Diablo 3 will no more be able to kill quickly with no +spell damage than a Barbarian or Monk could succeed with a junk weapon. A quote from Flux's post-Blizzcon 2009 Wizard write up explains this in more detail:
- When viewing the listed damage on each spell, it’s important to keep in mind that those are not absolute numbers, in D3. They’re much like weapon damage; modified by your wizard’s attributes, your character level, by other skills, and also by your equipment. Lots of types of items in D3 carry modifiers that boost spell damage, usually by a percentage.
- I found numerous wands, staves, and even pieces of armor with values between +5-25% spell damage, and that was just in the early stages of the game. We know nothing about higher level equipment bonuses, and it’s entirely possible that the D3 team is projecting high level characters to have +100%, +200%, or who knows, +500%, spell damage. If so they’ll be adjusting the base spell damage accordingly, which might make it look very low to our untrained eyes.
- Here’s an example from early in the game, that was mentioned in the Witch Doctor skills report. The skill tree listed damage on my level 2 Skull of Flame spell was 6-9, yet with a level 12 Witch Doctor my Inventory screen showed 19-29 for that spell. I don’t think I had more than +50% spell damage from equipment, so assuming the display values aren’t wrong (in D2’s infamous LCS style), there are considerable bonuses added to damage from character attributes, which are then further boosted by the spell damage bonuses on your equipment.
- This is all part of the D3 team’s plan to make all of the attributes useful to all characters, and to make more types of modifiers useful/essential to spell casters. We don’t know enough yet to judge how well it’s working, but it should be a substantial change from how spell damage was calculated and scaled up (or not) in D2.
The Witch Doctor skills are arranged into three distinctive skill tree categories:
All three trees have a combination of active and passive skills. Active skills are used in combat, while the passive skills boost or change the behaviour of the active skills.
The Zombie tree skills are focused on spells that summon Zombie and Zombie-related monsters. Several passive skills were revealed in the Blizzcon 2009 build of the game, that augment the Witch Doctor's minions.
The Spirit tree skills appear to be heavily focused on augmenting the Witch Doctor's own abilities, such as health & mana regeneration, and even staving off death by inducing invulnerability. So far there are very few offensive skills in this tree.
The Voodoo tree skills all focus more on the offensive aspects of the Witch Doctor; summoning Fetishes to fight, throwing acid on enemies or even summoning a whole army. Compared to the Spirit Skill Tree, there are very few passive skills in this tree.
The Witch Doctor class was unveiled at WWI 2008, together with the Barbarian. Besides a few name changes for spells, not much had changed for the Witch Doctor by mid June 2009.
You can find pictures in the Diablo 3 screenshot and picture gallery: