Here is the site with information on the Census Geography: Census Geographic Terms & Concepts http://www.census.gov/geo/reference/terms.html
American FactFinder provides access to data about the United States, Puerto Rico and the Island Areas. The data in American FactFinder come from several censuses and surveys.
For more information on how to use AFF, try one of these guides:
EJ Screen - EJ Screen, formerly the Environmental Justice Geographic Assessment Tool, allows users to combine map data from multiple sources, including Census, health, environmental and EPA monitored facilities to identify factors that might impact environmental health.
Illinois County Data Book - population, economic and election statistics for Illinois counties.
McDonough County Page - use these sites for other Illinois counties as well.