From PinataIsland.info, the Viva Piñata wiki
A Piñata garden is a paradise that has fallen into ruins. Your job is to restore it to its glory in whatever way you want, setting the stage for the garden to return to its lost splendor.
The garden starts out as a dirty brown landscape and you have to go about transforming it from a very basic level. As you start turning that brown patch into a garden you’ll attract piñatas who will help develop your land and, in turn, attract other piñatas. Running your own piñata garden may seem like a big task, but there are always Helpers to lend you a hand.
In your garden, the piñatas each have their own species-specific Housings in which they live and perform their Romance dance. If you work hard, your garden will flourish with many different kinds of piñata.