On the evening of Tuesday August 2nd, 2011, there was an excellent example of a "bow echo" thunderstorm. As discussed at the National Weather Service glossary, a bow echo is a radar echo that is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can sometimes develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation exhibits cyclonic (counter-clockwise) rotation.
It is easier to see how the bow echo evolves by animating radar imagery that moves with the storm, as is done with the animation below. The bow echo storm below had both a cyclonic (counter-clockwise) circulation crossing Southern Lower Michigan and an opposite anticyclonic (clockwise) circulation to the south that appeared just north of the Indiana border. An area of strong winds surged from the back of the storm to the front between these circulations (also known as "bookend vortices"). The wind surge can be seen as a light bluish green area accererating forward through the storm. This particular storrm produced spotty tree damage across Van Buren County.
Additional information about how the National Weather Service uses radar can be found at The National Weather Service JetStream Online School for Weather website.