These images are generated by geostationary satellites orbiting 22,000 miles above the equator looking at the United States. These include visible, infrared and water vapor images. The images are updated once an hour at about 30 after the hour.
This image is the equivalent of taking a black and white photo of the earth. The bright areas show where the sun is being reflected back into space as a result of clouds or snow cover. Clouds and snow show up white. The thicker the cloud, the brighter the color. Land surfaces show up as gray and ocean surfaces nearly black. The major limitation to visible imagery is that it is only valid during daylight.
There are 5 sectors for the visible image:
East - GOES east view centered over 75 west longitude showing eastern US and Atlantic Ocean.
West - GOES west view centered over 135 west longitude showing western US and Pacific Ocean.
Hemisphere - this is a pieced/merge view of both the GOES east and GOES west satellite views remapped to a Mercator projection.
US - this is a pieced view of GOES east and west remapped to a polar stereographic projection.
Regional - these are regional views remapped to a polar stereographic projection.