The Population and Housing Census is required by the United States Constitution decennially (every ten years) to enumerate the population for purposes of representation, that is, for the redistricting process. It is conducted in years ending in zero. Its questions are asked of all residents and for this reason it is sometimes referred to as 100% data; it is also known as short form or Summary (Tape) File 1 data.
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%). It was referred to as long form, sample or Summary (Tape) File 3 data. In 2010, the long form was dropped from the decennial census. Those data are now collected via the annual American Community Survey.
The subjects covered by the decennial census now are relatively basic counts of individuals by age, race & ethnicity, sex, households and housing units, but also includes description of household relationships, occupancy status and tenure (whether the residence is owned or rented). Data are available down to the smallest Census geography, the Census block.