Geographical divisions play a crucial part in the Census.  Collection of data is done by counting within a small geographical area and then combining the geographical units.  Census geographical divisions consist of familiar ones such as states and counties but also lesser known divisions such as Census Tracts and Census Blocks. 

NOTE: Some states do not follow the Census hierarchy faithfully, the New England states being the most obvious.  In New England, the cities and towns are the major political entities and counties are almost non-existent so for purposes of using the Census, the cities and towns are treated as "county subdivisions". 

The source listed below describes and explains Census geography in detail.