There are so many threads about it. And there are so many game reviews on it. Couldnt you just go read them >_> ...
Its like RPG + Maxis-Sim-Game. You start as a single-cell creature, evolving to a fish -> land creature , then to tribal stages -> city -> space. Along the way you collect creature parts and change your creature, do stuff with tribes/city etc etc...
it explains a bit of the game. but there are more longer review movies of spore on the internet. search arround a bit. but to know what this game really is u should just play it =p