It's a first-person shooter, in form only. You're a passenger on a cruise ship, and start out unarmed... everyone is playing a big "circle of death" game, though, and before the first round starts, you had better arm yourself. There are fire hatchets around, or golf clubs in people's cabins... Don't get caught wielding one of these in front of security, though, or you'll find yourself spending time in the brig!
Multiplayer online, or single player "arcade mode." It looks like it has some really neat mechanics; I plan on trying it a bit over the weekend, to see if it's worth $20. It seems likely.
[Edit: a friend gently reminds me that this game is rightfully called 'Killer' in its original real-life form]