Chao are small, cute creatures that appear in Sonic Adventure and Sonic Adventure 2. Depending on how you raise them, they will grow into an Angel Chao, Devil Chao, or Normal/Neutral Chao. There are 75 forms in all. A notable Chao is Cream the Rabbit's Chao: Cheese, which happens to be a Neutral Chao.
In Super Smash Bros. Brawl
A Chao appears as a Sticker and a trophy in the game.