A bioswale is an earth-friendly, bioretention landscape designed to filter, absorb and/or breakdown pollution from runoff water. They are found around large parking lots, parallel to roadways,buildings, and other large impervious areas, and are filled with vegetation, rocks and compost. 

Isn't this the same thing as a rain garden?

While bioswales and rain gardens aim to achieve the same goals, bioswales are designed to manage runoff from large impervious areas, and are greater in width, length and depth than rain gardens.