Relevant wiki articles:
http://wiki.openstreetmap.org/wiki/United_States_Road_Classification
http://wiki.openstreetmap.org/wiki/United_States_roads_tagging
map features:
http://wiki.openstreetmap.org/wiki/Map_features
http://wiki.openstreetmap.org/wiki/USGS_GNIS#Converting_GNIS_Nodes_to_Areas
http://wiki.openstreetmap.org/wiki/Category:United_States