AT-AT's were large, four-footed machines resembling mechanical elephants. Although their primary purpose was transportation of Imperial Stormtroopers, the walkers themselves were formidable. They possessed head-mounted blasters, armor too thick for blasters to penetrate, and they could walk over--and thereby crush--people and equipment.
At the Battle of Hoth in Star Wars Episode V: The Empire Strikes Back, the Empire used walkers to land beyond the limits of the Rebel Alliance's shield. General Maximillian Veers commanded the ground forces and advanced on the shield generator. Faced with such forces, Luke Skywalker devised a strategy to disable the walkers: his team of snowspeeders would use their magnetic harpoons and tow cables to entangle the walkers' feet, causing them to fall. Wedge Antilles and his gunner Wes Janson managed to disable one walker this way, Luke destroyed a second by cutting a hole in it with his lightsaber and tossing in a thermal detonator, and a third blew up when a speeder crashed into it.
Despite this, the Battle of Hoth was one of the most devastating losses for the Rebels in the entire Galactic Civil War.
Walkers were only one example of Imperial ground hardware. Other examples include the AT-ST or "chicken walker," the AT-PT, and the MT-AT.