Diablo, if you're not aware, is a game about stabbing the Devil and taking his clothes.
More seriously, it's a real-time isometric roguelike action-RPG with multiplayer. It's also really friggin' good. I might elaborate on this more when it's not 2 in the morning (it will be later still when I stop playing (really friggin' good)).
Out of courtesy to others, please avoid posting spoilers.
Update, 5/17/12: it is now 7:30 in the morning, and I have reached Act 3. I've died a total of 3 times, most of which could have been prevented by using a potion (gonna rebind pots to Mouse 3, methinks; Q is sort of awkward to hit from the number keys) or by not running into the middle of the enemies at the get-go (Diamond skin: feel invincible, then die very quickly when you're suddenly not). I have not as yet died on any bosses, major or minor. I'm level 26(ish), most of my gear has been and continues to be Rare quality and (mostly) level-appropriate; I have a level 3 Jeweler, a level 4(ish) Smith, and 42 storage slots. I don't consider myself a particularly good player as yet, but that hasn't kept the game from making me feel any less than godlike.