The census is required by the Constitution every ten years for purposes of reapportionment of seats in the US House. In addition to that simple purpose, by statute and regulation, a “long form” version of the census includes numerous additional questions for planning and survey purposes. Most people are only sent the traditional short form census questionnaire.
Officially, there is no “long form census” anymore. It was converted to the American Community Survey, which is a long form sent monthly to a few thousand households. The official 2010 Census was one form, with about 10 questions.