The All Terrain Scout Transport is a smaller and faster version of the AT-AT, or the Imperial Walker. These scout walkers are utilized primarily for reconnaissance and defense. They were built for speed, and therefore walk on two legs, like a chicken. That is why they are commonally called the Chicken Walker. Scout walkers provide covering fire for ground troops and are being used in growing numbers as direct reinforcements for front-line.
AT-ST's were used by the Empire during the Battle of Endor to suppress the Rebel's attempt to destroy the shields to the newly constructed Death Star. Unfortunately they didn't help much to stop the Rebels. With the help of the Ewoks, the Rebels defeated the Empire. Chewbacca and two Ewoks took over an AT-ST, and blew up other AT-ST's and Imperial ground troops.