Spore is several games in one, none of which are particularly impressive on their own. It's life sim meets strategy, where you grow a tiny organism with a little snappy mouth into a horrible monstrosity you made look like a penis because you’re hilarious. Eventually that penis will become a spacefaring species trying to traverse and spread throughout the galaxy.
Each stage of life in Spore comes with its own pretty unique style of play, some far deeper than others. Throughout, you must make decisions on how your species will evolve and thrive. Do you want to be a hippie herbivore, singing to befriend all your neighbours? How about a warmongering meaniehead, relentlessly murdering everyone who stands in your way, for no reason? Well, the choice is yours. I reckon it’s best played a bit in between the two extremes, as otherwise you’ll become far too powerful extremely quickly. But I’m not your dad. Do what you want.
If you haven’t played Spore, it’s honestly worth giving a shot. The game has lofty ambitions, particularly for 2008, and it is rather entertaining watching your disgusting monster rise up to become a super smart, diplomatic, spacefaring species (despite the extremely shallow pseudo-Civ penultimate stage). If nothing else, it's worth it if only for the couple of hours of entertainment you’ll get by jamming a mouth on your knees and singing beautiful ditties to the sensibly-designed fellow creatures.