The Census of Population and Housing, or Decennial Census actually is two censuses taken concurrently every ten years. The Census of Population, begun in 1790, counts the number of people and some personal, social, and economic characteristics. It is the oldest statistical product of the Federal government, and the first periodic census in the world.
The Census of Housing, taken since 1940, counts the number of residential dwellings and selected physical and financial characteristics. The Decennial Census is mandated by the U.S. Constitution, and by law (Title 13, U.S. Code), the Census Bureau cannot share the answers that respondents provide with anyone or any other government agency.
The Decennial Census' rich collection of data is also available at the state level, comparing population and housing unit changes by county. The Washington: 2010 Population and Housing Unit Counts was recently released by the U.S. Census Bureau and is available at Gallagher Law Library! Can't get to the library? Access the PDF version here!