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.
The water vapor image show an infrared band which is affected strongly by the presence of water vapor. Essentially, the image shows the altitude of the highest moist layer in the atmosphere. Bright areas reflect the location of high clouds either due to jet stream cloudiness or due to thunderstorm activity. The dark areas reflect the location of dry are at high altitudes. This is associated with dry air intrusion and sinking motion associated with high pressure systems. This image is a decent tracer of jet stream winds which will show up as bright streaks.
There are 4 sectors for the water vapor image:
East - GOES east view centered over 75 west longitude showing eastern US and Atlantic Ocean.
West - GOES west view centered over 125 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.