A rain garden is a garden where runoff water is directed. It's purpose is to catch rain as it runs down a yard or driveway, preventing it from polluting local watersheds. The water garden has native plants and flowers, many of which can be chosen for their ability to remove toxins.
If you're interested in this environmental and attractive attractive addition to your garden, check out this article about making your own rain garden. They walk you through each step, from your initial sketches, to choosing plants and controlling weeds. With suggestions on choosing your basin and dealing with landscaping challenges or restrictions, they are a great source of information.
Up to 70% of water pollution caused by storm-water runoff, this could be a great way to make an impact while beautifying your own yard.