MacLife has a great preview article on the different phrases of Spore as well as how your creature evolves thru each one. They also have a decent breakdown of how the game plays like and the total time it would take for them to become bored with each phrase.
Creature editors are the root of Spore’s creativity, a cross between Mr. Potato Head and Maya. After earning enough points and upgrade options–typically through eating enemies–you’ll be able to swap in an assortment of dozens of parts. Each trait directly impacts the creature’s performance in the world. Bigger arms pump up strength, extra legs boost movement, and a spiked tail adds an always-ready weapon. But the part roster includes horns, antlers, noses, mouths, and practically any other body part.
Those parts can go nearly anywhere. Instead of pre-planned animation, a procedural graphics engine figures out how your choice and position of parts affect the creature. Stick stocky legs at the back of a body, and the game makes the creature wobble. Add a third leg for awkward ambling. You control neck length, body shape, and nearly everything else, and the game automatically brings your creature to life.