They collect stormwater runoff, soak it into the ground, and dry out between rainstorms. In doing so, raingardens help retain soil moisture in our landscapes. Raingardens can fit into general landscaping or be used for butterfly, pollinator, and cutting gardens. They are also sometimes called rainwater gardens or downspout gardens.
This page has several references for download on how to build a raingarden in the Resources section below. The paper Introduction to Rain Gardens provides a summary of the key considerations for constructing rain gardens. It is a good starting point.
Two of my workshop presentations are available for download. Why Raingardens describes how we have altered the natural landscape, the consequences of those changes, and why backyard landscaping and raingardens are important. Raingarden Construction provides step-by-step instructions and photos that follow the topics in the introductory paper.Rain Garden Planting Plan is a sample garden plan for the central Midwest using native plants that also provide a butterfly and pollinator garden.
If you would like more detailed information, plant lists, and garden plans, consider ordering the book, The Blue Thumb Guide to Raingardens. It was written with Rusty Schmidt and Dan Shaw two talented landscape ecologists and designers, and I hope you will find it helpful.