These files contain data from the Decennial Census, such as total population, total housing units, median age, population 16 years and older, population over 65 years of age, race, relationship, average household size, and many others. They are available for a variety of geographies.
Access this data online by creating a personal account or click the drop down arrow next to the Add Data button in ArcMap. Select Add Data from ArcGIS Online... and search for "census." Most census data has been posted by ESRI. Some data has been posted by users.
The LTDB provides estimates using 2010 boundaries for a standard set of variables from 1970 through 2000. The "full count" files include the key variables from Census 2010 (race and Hispanic origin, age by race and Hispanic origin, household composition, and housing occupancy and tenure), with comparable variables from the censuses in 1970-2000 and adjusted to 2010 boundaries. The "sample count" files include a wide range of other variables based on sample data -- the one-in-six samples from the decennial census in 1970-2000 and the sample data from the American Community Survey for 2006-2010. These include variables such as socioeconomic characteristics that are widely used by researchers.