This image of North and South America city lights was created with data from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS). Originally designed to view clouds by moonlight, the OLS is also used to map the locations of permanent lights on the Earth's surface.
The brightest areas of the Earth are the most urbanized, but not necessarily the most populated. Cities tend to grow along coastlines and transportation networks. Even without the underlying map, the outlines of the continents are still be visible. The United States interstate highway system appears as a lattice connecting the brighter dots of city centers.
See also the rotating Earth animation VSS00012.