The United States Federal Census has been taken every ten years, since 1790. Most people know that. However, what many fail to realize is that no two Census years are identical in the information that was collected. As researchers, it's so important to familiarize ourselves with the questions that were asked in each census year; and, not only that, but to also be aware of the many supplemental census documents that are available from various years. Being armed with this information will move us forward in our research in a more informed way, and can help to eliminate the frustration of trying to find certain valuable nuggets of information which might be just one quick click away.
For a very comprehensive listing of what is included in each census year, as well as what other schedules (such as slave, mortality, agricultural and more) are available, please visit this excellent source from The Family History Daily: The Ultimate Quick Reference Guide to the U.S. Census for Genealogy Although I did find one error - that the 1850 and 1860 Slave Schedules are exactly the same - this guide includes a wealth of information about what is included in each enumeration, for each census and supplemental schedule. Though all data collected in census documents needs to be corroborated from other sources, the U.S. Census is a valuable starting point for most who engage in genealogical research.
Additional sources for understanding census rules and enumeration can be found at these links:
*All sources are linked in the post, above.