The United States Census is a population count carried out in the United States every 10 years since 1790, under a mandate from the Constitution. The results of the Census are used in a variety of ways, ranging from the allocation of Congressional seats to determinations about federal funding. The goal of the United States Census is to count every single person in the United States, and to gather basic statistical information about the people living and working in the United States.
As you might imagine, a project of this scope requires some serious organization. Since 1903, the Census has been administered by the United States Census Bureau, a government agency which also conducts surveys during off-years. The Census Bureau has a huge staff of agents which is increased during Census years to cover communities on foot, ensuring that United States Census forms reach as many people as possible.
Two forms are used in the United States Census: the long form, and the short form. The short form has a few basic demographic questions, asking the name, age, and race of the respondent. Respondents are also asked to describe their relationship status and housing tenure. In the long form, a more detailed set of questions involves education, ancestry, employment, and so forth. One in every six people is asked to fill out a long form, with the resulting data sample being used in a variety of ways.
Along with counting citizens, the United States Census also covers legal residents and illegal immigrants. It includes the homeless, people in institutions, and other people with unusual housing arrangements. Due to concerns about confidentiality, specific census information is kept sealed for 72 years, although general United States Census data is available immediately. This confidentiality measure is designed to encourage people to respond to Census forms without fear of legal consequences.
Many people use the statistics from the United States Census, and the Census Bureau hosts an excellent website which presents Census data in a variety of innovative ways. Detailed data from 72 or more years ago is also readily available, and it can be extremely useful for people who wish to research family ancestry and other topics. Searches of Census records are also used by historians to learn more about the people they profile, as such records can be used to find out when someone moved to a new city, married, or had children.