The Distant Early Warning Line was a system of radar installations from Alaska to Iceland and Greenland designed to watch for a Soviet Nuclear air attack coming over the North Pole to Canada and North America. It was made up of 63 stations over a 10,000 kilometer swath.
As you can see from this azimuthal equidistant projection map (thanks google!) you can see that the shortest route between the USSR and the USA was over the North Pole. Faced with the technological obsolescence of the Mid Canada line and Pinetree line of radar stations the Governments of Canada and the USA agreed in 1954 to jointly build a third line of radar stations, this time running across the high Arctic. The line would run roughly along the 69th parallel, 200 miles or 300 kilometers north of the Arctic Circle. America agreed to pay for the construction of the line with Canadian labor, and Canada agreed to have joint US- Canadian staffing in many of the stations. The stations used a number of long-range Pulse radars called AN/FPS-19. They were interconnected by a series of secure radio systems, often relying on tropospheric scattering to both boost the signal and ensure security.
The line consisted of Large stations with a sizable staff and amenities spaced out with intermediate stations with only 3 people ( Chef, Mechanic, and Chief) in between. There also were small unmanned "gap fillers" that were checked by aircrews only every few months during the summer. The gaps between the stations were watched by the directional AN/FPS-23 Doppler radar.
With the introduction of ICBMs and submarine launched missiles a few years after it was completed, the line lost became somewhat obsolete. A number of stations were decommissioned, but the bulk were retained to monitor potential Soviet air activities and to assert Canada's sovereignty in its Northern territory.
In the 1980's, the more capable of the DEW Line stations were upgraded with new technology and automation was increased. These sites were merged with the newly-built stations into the North Warning System and became a main part of NORAD, headquartered in Cheyenne Mountain, Colorado.
In 1990, with the end of the Cold War, the United States withdrew all their personnel and turned full operation of the Canadian stations over to Canada, while retaining responsibility for NWS sites in Alaska and Greenland.