"AT" stands for Alpine Touring.
The idea is to have an alpine, ski boot, with a climbing sole (and typically a lever that releases the stiffness in the ankle-cuff for walking), and a binding that can have a free heel for climbing or aproaching. Using a climbing skin on the bottom of the ski (a synthetic velvet with strong grip in one direction and none in the other) allows you to use the skis to climb uphill, or simply to grip for flat touring. This setup is usually used to access backcountry/big mountain terrain.... in the wilderness as opposed to a resort. When an AT skier reaches the top of a mountain, he removes his climbing skins, locks down his heel, and skis in normal alpine fashion.
FYI - the magazine that I work for covers all things backcountry;www.earnyourturns.com