From the game guide

Content sharing

One of the greatest aspects of Spore is that nearly everything you see will be created by other players. From the creatures populating worlds to the very world itself — everything is asynchronously pulled from a central database.

Asynchronous sharing

Asynchronous sharing of content merely means that your computer will upload and download new creatures in the background while you are playing the game. You won’t need to press any buttons that “sync up” your game, or worry about not having any creatures in your world - it’ll all just happen in the background.

Your world is created by other players

This is one of the most exciting aspects of the game - everything you experience in the world will be created by other players. That means that when another creature attacks your creature, the attacking creature will be a creature that maybe your friend down the street created. This will keep the game fresh and very different for each person playing the game.

Sporepedia

All of the content in the game that you encounter will be stored in something called the Sporepedia. The Sporepedia is kind of like a collection of trading cards with various objects on them. For example, a Sporepedia card for one of the creatures you make will have a small shot of your creature, it’s basic statistics as far as intelligence, speed, strength, etc and how it has fared in other players games (whether it loses a lot of fights or wins many).