The Decennial Census is required by the United States Constitution to enumerate the population for purposes of congressional representation, that is, for the redistricting process. It is conducted in years ending in zero. From 1940 to 2000, a long form questionnaire was included in the Population Census to collect more detailed data from a sample of the population (about 17%). Those data are now collected via the annual American Community Survey.
Here is a reference guide to the Census providing definitions of terms and geographic areas. define geographic areas. Find reference information such as FIPS and ANSI codes, census tracts, etc.
In 2002 the Saturday Evening Post published an historical summary of the changes in the Census form: Measuring America: The Decennial Censuses from 1790 - 2000
In addition to the Decennial census the Census Bureau conducts many surveys.
The American Community Survey (ACS) began in 2005 as a nationwide survey conducted by the Census Bureau that replaces the “long form” in the decennial census. The ACS collects demographic, housing, social, and economic data on a three million household sample of the United States each year. The Census Bureau releases ACS data on a rolling basis, with three different files covering a one-year, a three-year and a five-year release.
Be careful about comparing ACS data with the decennial census. The Census Bureau provides guidelines:
The 2010 Census shows the number of people who live in the U.S. and the American Community Survey shows how people live.