From Blizzard Entertainment:
Diablo 3 takes place on Sanctuary, a world of dark fantasy. Unbeknownst to most of its inhabitants, Sanctuary was saved some 20 years ago from the demonic forces of the underworld by a few brave and powerful heroes. Most of those warriors who directly faced the armies of the Burning Hells-and were fortunate enough to survive-went mad from their experiences. And most of the others have buried their haunted memories and pushed the horrors from their thoughts. In Diablo III, players return to Sanctuary to confront evil in its many forms once again.
Choose from five powerful character classes, including the barbarian, witch doctor, wizard, monk, and demon hunter;
Numerous indoor and outdoor areas detailing new regions in the world of Sanctuary;
Vast assortment of fiendish monsters, with unique attack patterns and behaviors;
New runestone items that let players change how their abilities look and function, allowing for nearly limitless skill customization;
Four different difficulty settings-Normal, Nightmare, Hell, and Inferno-progressively increase the challenge and reward players with more powerful armor, weapons, and runestones;
Brand-new 3D graphics and physics engines enhanced with spectacular visual effects;
Interactive environments with dangerous traps and obstacles, and destructible elements;
New quest system and character-customization options for the ultimate action RPG experience;
Multiplayer functionality over Battle.net with support for cooperative and competitive play.