From the review: "What is truly remarkable about Spore is the grand level of customization that the game gives you in designing each and every aspect of your creature. You can design how it looks, right down to the very tiniest details. You can give it any amount of arms, legs, eyes, mouths, or torsos that you desire. In addition, you can design the homes they live in, the vehicles they fight with, and the airplanes they fly in. You can even eventually shape the very planet they live on, and can terraform other planets to make them suitable to live on for your species.
What's even better is that every time the game starts up, the game logs on to the Spore central server and downloads other creatures designed by other users. This means that all of the other creatures that you face throughout the various stages of evolution are all user generated. Even Will Wright himself has described Spore as a "massively-single player game."