The general population is familiar with ZIP codes, county and state boundaries. However, the Census also offers statistics in other useful geographies (e.g., NECTA, County Subdivisin, Tract, etc).
But, understanding the hierarchy of Census geographic entities is not intuitive. The Census produced a diagram explaining the levels geography (http://www.census.gov/geo/reference/pdfs/geodiagram.pdf) but more can be done.
How about an interactive application that explains how the different Census Geographic Entities interact?