A subdivision of a census tract, a Census Block is the smallest geographic unit used by the Census Bureau for demographic tabulation. In urban areas, a Census Block is roughly the size of a city block wherein the blocks usually correspond to individual city blocks bounded by streets, but blocks – especially in rural areas – may include many square miles and may have boundaries that are not streets. Blocks are renumbered for each census, so Census Block 2345 in 1990 is not necessarily the same as Block 2345 in 2000, but an “equivalency table" is published. Because they are based on the road network, once the road network in a developed area is established, block boundaries are consistent from one census to the next.
A Census Block Group is a geographical area consisting of a group of Census Blocks. Block groups do not have consistent boundaries (or identification numbers) from census to census and are intended primarily for Census Bureau data processing purposes. Aggregation of Block Groups create Census Tracts and are used by the Census Bureau for demographic tabulation.
Census Tracts are combinations of block groups. Census Tracts are relatively permanent geographic divisions of a county defined for the tabulation and presentation of data from the decennial census and selected other statistical programs. The primary goal of the Census Tract is to provide a set of nationally consistent small, statistical geographic units, with stable boundaries, that facilitate analysis of data across time.
A statistical entity, defined for each decennial census according to Census Bureau guidelines, comprising a densely settled concentration of population that is not within an incorporated place, but is locally identified by a name. CDPs are delineated cooperatively by state and local officials and the Census Bureau, following Census Bureau guidelines. There are no population size requirements for CDPs.