You may only know about the U. S. Federal censuses which have been taken every ten years since 1790 which are a wonderful source of information in tracking and learning about your ancestors. What you need to investigate is the data available in the numerous states censuses.
The beauty of using a census done in an individual state, at a certain time period, is that it offer more information than was placed on the Federal censuses and also cover a different time period. Those states who did do their own state census completed them in between the decades of the Federal censuses. So with a 1870 Federal census, a state could do a 1875 census, five years after the last Federal and five years before the next Federal census. Unfortunately, not all states did their own state census at any time period. Some states who did complete a census may have only done it once or twice or skipped more than five years at a time. Delaware only did one in 1782, Maine only in 1837 and North Carolina just once in 1786. Then there are about nine states who never completed a state census, or at least one that was ever found.
Such state censuses that have survived are held in the state’s archives or state library. On microfilm are available state censuses with the Family History Centers of the Church of the Latter-Day Saints and many fee subscription databases also carry the state censuses.
If you have relatives from a few select states which did multiple censuses you are fortunate. Places like the State of Washington, Georgia, Oregon, Iowa, Florida, Mississippi, Alaska and New York have many years covered, sometimes even before they became officially a state. A couple, such as Florida and South Dakota, even have censuses beyond the Federal 1940 census which is the next census available starting in April 2012. The District of Columbia has three censuses of the district covering 1803, 1867 and 1878; all interesting periods in Washington, D.C.’s history. All which can be found on the FamilySearch.org web site.
Not all the state censuses are online to date, however many have been placed on the Internet and others are being prepared. Do a ‘Google’ search by placing the state name along with the phrase ‘state census’ to see what is available online. Also check the Family History Centers (online Family Search.org) and the individual state archives to see what is available for any specific year.