According to the USGS, a "Geographic Information System (GIS) is a computer system that analyzes and displays geographically referenced information. It uses data that is attached to a unique location."
National Geographic notes "GIS can show many different kinds of data on one map, such as streets, buildings, and vegetation. This enables people to more easily see, analyze, and understand patterns and relationships."
Geospatial Data Gateway is from the USDA Natural Resources Conservation Service and provides links to a variety of federal government data portals
MAGIRT is the American Library Association Maps & GIS Librarians Round Table portal to the world of Maps and GIS. Check out their data site
Free visualization tools
Those used in CWU Geography GIS Course site license for all ESRI products(ArcGIS primarily).