What actually kills you when you "die of thirst"?

OK imagine I get knocked out by burglar and he handcuffed to the radiator (I hate clich� so I'm not having a bloody desert island as the hypothetical) presumably if I lived alone no one would come looking for me and I'd die of thrust... but I've always wondered what does that mean exactly, I know every cell needs water but you die of thirst before you get that dry so basically what goes first? is what I'm asking, what vital bodily function can go the lest amount of time without water?