NORAD's original mission was to coordinate U.S. and Canadian
Air Force assets to fend off any intrusions into either nation's airspace. The primary threat envisioned was a massive
Soviet bomber attack; in which case,
interceptors would be
scrambled from Canadian and U.S. northern bases and vectored to their tagets by
ground controllers at
NORAD. Their headquarters is the (in)famous
Cheyenne Mountain, as seen in the above-noted movie
Wargames.
Later, as ICBMs began to become operational in the USSR, NORAD was given the additional mission of providing early warning for U.S. nuclear forces and civil defense organizations in the event a missile attack was launched against the United States. The existing DEW Line was not able to detect missile tracks; a new system, BMEWS, was deployed in northern Canada and Alaska to provide this capability. As other threats arose, NORAD was tasked with these additional missions, for example SLBM warning radars on the U.S. coasts.
NORAD operations were closely linked to SAC (which is now decommissioned) ops; SAC shared several facilities with NORAD including the Cheyenne Mountain control center.