Today's Image of Mars is of an inverted crater surrounded by dark dunes in Arabia Terra. The dark dunes surrounding the inverted crater are comprised of basalt, a black igneous (volcanic) rock commonly found on Mars.
Inverted craters are formed in much the same way that inverted riverbeds are created. Sediment becomes deposited in the crater and packed down, usually in the presence of water. This make the material much denser and more wear-resistant than the surrounding terrain. Over the course of millions and billions of years wind erosion ended up wearing down the surrounding terrain, but since the sediment packed into this crater was more resistant, less of it was eroded. This created the inverted look that we see in the image below.
Clicking on this image will take you to a captioned HiRISE image that provides context for this inverted crater.