"A raingarden is simply a garden with a depression that is designed to catch rainwater runoff in your yard, growing plants that don't mind getting partially flooded on occasion.  Raingardens provide wildlife habitat and an opportunity to create beautiful landscaping.  And, by soaking up rain where it falls, rangardens slow stormwater runoff, help prevent erosion and remove pollutants in the process."
— From "The Blue Thumb Guide to Raingardens", by Rusty Schmidt, Dan Shaw, and David Dods, available for purchase online at Amazon, or in person at a variety of locations, listed on Blue Thumb's website. 

Here are some great online resources:
• From the University of Wisconsin Extension Publication Library: Rain Gardens: a how-to manual for homeowners
•  Blue Thumb 

Additionally, if you would like to visit a raingarden in your area click here.