Top devs on the game said that fighting games, primarily Street Fighter, informed their design of the monk, who's essentially a DPS class that deals a lot of hurt to enemies but isn't particularly hearty himself. During the Diablo III class panel here, the devs mentioned that they looked at not only fighting games but also World of Warcraft's rogue class and the assassin from Diablo II in designing the monk's fast-paced, combo-oriented fighting style. He's described as a "good contrast" to the game's burlier, tank-like barbarian class.
Here's an incredibly long video of the monk kicking some tail, which you can watch some or all of to get a good idea of what he's about.
I hopped on a demo station for about an hour and got a pretty good feel for the way the monk plays. A number of his abilities are designed around three-step combo sequences, including two on display here called Exploding Palm and Crippling Wave. Palm is a straight damage-dealer that delivers successively more damage with the first two hits and then adds a bleeding damage-over-time effect on the third hit. Wave has a set of debuffs on it that slows enemies with each hit and then strikes multiple enemies on the third hit with an area-of-effect attack.
What makes the monk fast and dynamic to play is that you can mix and match these two combo-based abilities on the fly. So you could do a quick first hit with Crippling Wave to slap on a debuff, then get direct damage with the first two hits of Exploding Palm, refresh your debuff with another Wave attack, then go for the final DOT from the third Palm hit. Of course, you can also just hammer on mouse button one to keep doing the same basic three-hit combo over and over, but the more you customize your attack patterns, the more efficient your combat prowess is going to be. Since the monk isn't the most durable character in the game, you'll probably want to kill enemies as fast as you can to maximize your survivability.
The monk isn't all combos; he has plenty of direct-use abilities too. One of them which you see to the right there has him dashing between multiple enemies so quickly he appears to teleport, delivering a strike each time he passes one. There are some defensive abilities too, including one that deflects all melee attacks for a few seconds, and another that temporarily blinds nearby enemies and drastically lowers their chance to hit you. The monk seems like a versatile combat class with a decent amount of flexibility, from the limited set of abilities Blizzard has shown so far.
If you're wondering who this guy is, some of the game's top devs said during a panel that the monk is a spiritual warrior on the side of good, and belongs to a highly structured theocratic society dominated by the church. He pretty much exists to act as the muscle for the church and carry out its heavy lifting. Watch this cinematic trailer if you want to know more about who the monk is and where he comes from.
As of today we have the barbarian, witch doctor, wizard, and now the monk in Diablo III. There's one more character class for Blizzard to reveal, but since it's assuredly going to be a matter of years before this game is on store shelves, they've got plenty of time to prep that last announcement.
In the meantime, I got to speak to some of the top guys behind Diablo III, so take a look, won'tcha?