It stands for United States Geological Survey, and Carolyn L. Barkley wants you to know how important a site it can be for genealogists:
Now that you’re curious, here’s the link for the USGS website: http://www.usgs.gov/
Note the Science Topics section in the right-hand navigation bar. Earthquakes, Volcanoes, and Wildfires are just a few of the topics you can explore. Oh, and they're the folks who create those wonderful topographic maps, by the way...