The Chao Garden, a staple of the Sonic Adventure experience. First introduced in 1998's Sonic Adventure for Sega Dreamcast, the Chao Garden is a virtual pet simulator where you raise little creatures called "Chao" and force them to compete in races. Chao are upgraded by giving them animals/chaos drives, and by feeding them food. It's became famous for how fun it is, and whole communities separate from the Sonic Adventure 1/2 community have sprung up around this feature.

A lot of the Chao Gardens out there, big and small, usually aim to explain the basics of the Chao Garden, what it is, how to get there, what to do with Chao, how to raise them, where to find things to upgrade your Chao, how to race Chao, etc. Information between them might be slightly different, but they all basically talk about the same thing. Once you've seen one, you've seen them all.