Put simply, the US Census is an exact count of the entire US population used to properly draw the lines for congressional districts and distribute federal government funds. The mandate to properly enumerate the entire US population was written in the US Constitution by the founding fathers. Article I of Section 2 of the US Constitution states, "Representatives and direct taxes shall be apportioned among the several states which may be included within this union, according to their respective numbers . . . The actual enumeration shall be made within three years after the first meeting of the Congress of the United States, and within every subsequent term of ten years, in such manner as they shall by law direct." To view the full text of Article I Section 2, view the full text of the Constitution on the National Archives page.
The very first US Census took place in 1790 and had only 6 questions. Now the US Census has many more questions and data collection is not limited to every 10 years. The Census Bureau conducts both the Decennial Census, an exact count of the US population every 10 years, and the American Community Survey, a sample of the US population every year. The data from the US Census Bureau has many interesting applications for research and discovery. Once census data is properly processed it is published by the government for free online for anyone to use.