The Decennial Census:
The Decennial census is a constitutionally mandated count of the population of the U.S. which was first conducted by the government in 1790 and has been done every ten years since. The count is used for apportioning Congressional seats, as well as many other purposes, such as demographic research, allocating funding, and informing public policy.
The Census has changed greatly over time. Though the first Census in 1790 did little more than count population, over the years it has grown to include many other types of information. Many questions and terminology (e.g. racial categories) have changed over time as well.
The Long Form and the American Community Survey:
Over time Congress became concerned that there were too many questions. In 1940 the Long Form was created as a way of finding out more detailed information (e.g. questions on income and educational attainment) from a sample of the population. The long form of the Census was sent to 1 in 6 households. The 2000 Census was the last time the long form was used. Since 2000, the American Community Survey (ACS) has replaced the long form. The ACS is an ongoing survey of a sample population that will provide us with detailed and more current information. ACS data comes in 1-year, 3-year, or 5-year estimates.