The U.S. Census counts every resident in the United States, and is required by the Constitution to take place every 10 years. In between, the Census Bureau conducts American Community Surveys to keep the data up to date.
Go to the Census web site to obtain data from the Census, and use American Factfinder to access the available data. In American Factfinder you can find data in the form of maps, tables, and reports from a variety of Census Bureau sources. Look at the American Factfinder help page for additional information.