The United States Census Bureau is the leading source of quality data about the nation's people and economy. It operates many data collection, analysis, and dissemination programs, including the Population & Housing Census (or Decennial Census), Economic Census, Census of Governments, American Community Survey, Economic Indicators, and more. For a full list of topics covered, see the Census Bureau's A to Z List of Subjects.
Population & Housing Census/Decennial Census
When people talk about the U.S. Census or census data, they are commonly referring to the Population & Housing Census, which is also called the Decennial Census. This program, which takes place every 10 years and is mandated by Article I, Section 2 of the Constitution, counts every resident in the United States using a mail questionnaire and follow-up phone and in-person survey techniques. The data collected are used to determine the number of seats each state has in the U.S. House of Representatives and to apportion federal funds. What questions were asked? See the Historical Index of Census Questions
American Community Survey
Prior to Census 2010, detailed information about the U.S. population was collected using a "long form" sent to a 1-in-6 sample of households. This form was discontinued for Census 2010, so now we rely on the American Community Survey to provide the detailed information we used to get from the long form. The American Community Survey is an ongoing survey and data is released in 1-year, 3-year, and 5-year chunks.
What questions were asked? See the ACS Questionnaire Archive