In response to a question I have the following definitions!
Alpine Touring. Equipment and technique designed for big mountains with long ascents and descents. Skis are like downhill skis, heavy long or medium long boards, boots you can walk or climb in. Ski up with skins, heels free, and heel plugs, no postholing. Get on top, drink wine, eat cheese, get laid. Lock down your heels and flip the heel plug down. Ski down the couloir like Jean-Claude Killy, yodeling all the way. You're in heaven- the best of both worlds. If you want to go slow and cautious, you can ski down with skins (bad form but what the hell, you can still yodel.)
Same as AT, basically. French for "can't tele."
Tele is a turn technique, not a kind of skiing or ski. Good for the backcountry. It can be done on any kind of ski setup that allows you to lift your heel. It's a graceful turn, kind of a swooping motion. Impresses the hell out of granola chicks.