The American Community Survey is a statistical sample survey, distributed to approximately 2.9 million households per year starting in 2005. It forms the basis of Census Bureau statistics on a range of demographic, economic, social, and housing characteristics. Before 2005, a supplementary survey was distributed to a subset of households as part of the Decennial Census every ten years. The ACS provides period estimates. That is, it estimates average characteristics of people over 1-year, 3-year, and 5-year periods, rather a point-in-time estimate like the 2010 Census.
For more information see "An Overview of the American Community Survey" from the Census Bureau.