The Decennial Census
Regarding the counting of the U.S. population to determine the number of representatives in the House of Representatives, Article I Section 2 of the U.S. Constitution states that "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."
The first Decennial Census was counted in 1790 and has been counted and reported every 10 years since then.
American Community Survey
This U.S. Census survey began in 1996 and reports annual estimates about communities in the U.S. The importance of this survey and its published datasets rose in prominence in the mid to late 2000s. This is beacuse the additional questions asked each Census since 1960 through sampling (through the use of a Long Form) was not done in 2010. Many valuable research criteria, such as income and various housing characterics, were not asked during Census 2010 and the American Community Survey replaces the Decennial Census as the source for this data.