Catz is a game for GBA, Wii and PC, although PC and Wii versions are a little different from the one this guide it featuring, as this is for the Game Boy Advance version. Basically, you need to look after a cat for a certain length of time before they grow up, buy them food and toys, and raise their levels.
At the beginning of the game, you will have the choice to start a new game, or watch the credits. You meet a girl in the park and she has a cat, and you beg your parents for one. After they say yes, you can go to the pet shop and choose which breed of cat, and then take it home. For your first cat, it will grow up after 2 weeks, but if you are good at raising their levels, you will have longer to play with them.
Every Sunday, the girl will come and check on you and make sure you're taking care of your cat. From the start menu you can also view an album where memories of your past kitties are kept!
You can go to the pet store to get toys, food, treats and lots of other things, but you cannot buy cats apart from when starting a new game or have looked after your cat for the period of time you had.