If you believe the movies it\'s a bunch of people in an underground bunker somewhere remote (in a mountain I believe?) monitoring stuff and giving directions in case someone/something threatens the US.
EDIT: It\'s in Colorado Springs, Colorado.
EDIT2: Under the cold war they\'d monitor the sky for nukes and retaliate if the Russians sent nuclear missiles. I guess they still do.
EDIT3: The North American Aerospace Defence Command (NORAD) is a binational military organization formally established in 1958 by Canada and the United States to monitor and defend North American airspace. NORAD monitors and tracks man-made objects in space and detects, validates and warns of attack against North America by aircraft, missiles or space vehicles. NORAD also provides surveillance and control of the airspace of Canada and the United States.
NORAD uses a network of ground-based radars, sensors and fighter jets to detect, intercept and, if necessary, engage any threats to the continent.
NORAD\'s surveillance and control responsibility for North American airspace is divided among three NORAD Regions -- one in Canada, one in the continental United States and one in Alaska. Data from radars and sensors based in Canada are compiled and analyzed at an underground complex located at Canadian Forces Base North Bay, Ontario, then forwarded to Canadian NORAD Region Headquarters at CFB Winnipeg, and the NORAD command and control centre in Colorado, as necessary.
The North Warning System (NWS) provides surveillance of potential attack routes via Arctic airspace. The NWS consists of 15 long-range radars (11 in Canada, four in Alaska) and 39 short-range radars (36 in Canada, three in Alaska) along the northern edge of North America. The state-of-the-art radars form a 4,800-Km-long and 320 Km-wide "tripwire" stretching from Alaska to Newfoundland.
Airborne radar coverage is provided by the E-3 AWACS aircraft when necessary. Canada contributes military personnel to AWACS operations. The United States Air Force AWACS provides a significant improvement over ground-based radar stations, and extends the perimeter radar system in times of increased alert. AWACS aircraft can detect targets from about 580 Kms, then guide Canadian or U.S. aircraft to visually identify unknown aircraft.