Since 1790, and every 10 years the United States has collected census information to keep track of population growth and other items. The first Census to actually record names of members in a household was the 1850 Census, and in 1860, relationships (son, daughter, etc) were added. The last viewable US Census was the one done in 1940. The 1890 Census was mostly destroyed in a large fire in January 1921.
Many States also recorded their own census as well. England also started keeping a population census in 1801 and every 10 years thereafter. The first 4 census records were mostly for statistical purposes, though the 1841 census started to record names, ages over 15, etc.
So much information can be gleaned from these records about our ancestors and then the descendants of those ancestors. It's a wonderful experience to view information about them from these paper records.