It's basically takes NWS radar and allows you to add geographic info to your selected maps and gives the user the capability to zoom in - right to the street level like those fancy Vipir systems used on TV. Not quite as sophisticated but it's only about $70. It shows a host of info like storm attributes (hail size, shear/rotation), storm relative velocity, storm rain, cell tops and a whole host of other good stuff.
Here's a screen shot of GRlevel3 radar taken at 11:19 am from Columbus AFB, MS.
Notice that there was a TVS indicated - tornado vortex signature with this thunderstorm complex moving into Middle TN. The TVS has not remained constant but there is strong rotation (a meso) indicated at the tail end of this complex on the MS/AL border (as of 11:51 am - click on pic to enlarge)