Radar is a minimap replacement addon. It hides the terrain part of your minimap, collects buttons in a dock frame, and puts the rest in the center of your screen, to help you see the directions of arrows and locations of icons relative to your character. By default the button dock is placed at the top of your screen and shown only on mouseover; this can be changed in the interface options pane.
It's suggested that you enable minimap rotation for this to look right - go to Interface -> Display and check "Rotate Minimap", or enter the command /run SetCVar("rotateMinimap", 1).
6.0 fixes; interface options added; some icons replaced