There are three gardens total (note that there is NO Chaos Chao Garden, or any other "special" garden). You start out with the Neutral Garden and can obtain the Hero and Dark Chao Gardens (look below for more info). Each garden is the same size (really they are, so stop annoying me about it), can hold 8 Chao, and when first unlocked, 2 Chao eggs can be found near the entrance.

The Neutral garden is the main garden (remember that you start with this one). To unlock it, you need to first find the Chao Key in the action stage. It is a simple garden with a little pool next to a mountain, in this mountain can be found the Chao Stadium! (see the Chao Stadium page for more) You can dig up pumpkins with either Rouge or Knuckles, and sell them for 50 rings or give them to a Chao to wear on they're head.

This garden is sometimes known as "Chao Hell" and is unlocked after you create a Dark Chao. It has a pool with blood red water, and many tombs and graves, making it very different from the other gardens! Here you may dig up skulls instead of pumpkins for your Chao.

This is the most peaceful garden and is also known as "Chao Heaven." It is unlocked after you create a Hero Chao. The Hero Garden's pool has a fountain in the center of it and a stone pavilion hanging over edge. Just like the Neutral Garden, pumpkins can be dug up here.

Personally, I prefer the Dreamcast gardens. The Dreamcast Hero Garden has a smaller pool but has a river runing through the garden, and the Dreamcast Dark Garden has a big mountain on one side (I don't think the Neutral Garden changes much). Also, the gardens are bigger on the Dreamcast. So, doesn't it sound cooler? To me, and many others, yes! But before you go buy a Dreamcast off Ebay, let me just say that Chao on the Dreamcast appear to be buggier. Here are some screenshots I took of the Dreamcast gardens:

Neutral Garden 1
Neutral Garden 2
Dark Garden 1
Dark Garden 2
Hero Garden 1
Hero Garden 2