Male Monk concept art.
The Monk is a melee fighter class that specialises in fast combo attacks and benefits from his expertise in crafting Runic auras.
The Monk in Diablo III bears no relation to the non-canon Monk class that was added to the original Diablo in the Hellfire expansion pack, as Diablo III Community Manager at Blizzard, Bashiok, explained in reply to a forum post relating to the Monk's origins:
"it isn’t the same class. We aren’t taking an old class and updating it. The monk from Hellfire, and in fact the story and content of Hellfire, doesn’t exist as far as game lore and story is concerned going into Diablo III.
They happen to share a name, the same as all of the other monk classes that have existed in all other RPG’s since the beginning of time. It doesn’t mean they’re related kit/story/flavor wise."Hellfire