When considering data from the Census of Population and Housing, it's important to know whether the questions were asked of all respondents or just a sample.
100% (Short Form) and Sample (Long Form) Questions
The 1940 census provided the prototype for the census today. It was the first census to develop and apply sampling techniques, thus creating the short form or 100% questionnaire and the long form, or the sample. Since then, the Census Bureau has continued to refine its sampling techniques.
The questions asked on both forms have changed over time; they reflect the changes in our society and the need for statistical data mandated by Federal legislative programs. See the Major Differences in Subject-Matter Content between 1990 and 2000 Census Questionnaires for the changes in that ten-year period, and for a longer historical perspective, Measuring America: The Decennial Censuses from 1790 to 2000 (PDF), which includes the questionnaires and instructions to census takers. The Census Bureau also makes all decennial census questionnaires available online.
American Community Survey (ACS)
As of 2010 the ACS has replaced the decennial census long form, so that estimates of data on demographic, housing, social and economic characteristics will be available more frequently than once every ten years. Data will be provided annually for states, cities, counties, metropolitan areas, and population groups of 65,000 people or more. For smaller areas, it will take three to five years to accumulate sufficient sample to produce data for areas as small as census tracts. If you are wondering whether to use the 1-year, 3-year or 5-year ACS, the U.S. Census Bureau has provided data users with some guidance here. See ACS Data Releases for details and release dates, and How to Use the Data: Errata for notes on problems with the data and the status of their correction.
Where to get ACS data:
American Community Survey: Data & Documentation (Census Bureau)
You can also find ACS data in many of the same locations as the decennial census data, including SimplyMap and SocialExplorer.
Estimates & Projections
Estimates and projections provide a limited amount of data for years between the decennial censuses.
The Census Bureau publishes total resident population estimates and demographic components of change (births, deaths, and migration) each year. The estimates are also published by age, sex, race, and Hispanic origin for the nation, states and counties. Estimates usually are for the present and the past, while projections are estimates of the population for future dates.
Where to get population estimates and projections data: