United States Federal Censuses have been taken every decade since 1790, however the record of the 1890 census was almost entirely destroyed in a fire at the US Department of Commerce in 1921. Most good libraries in metropolitan areas will have copies of the US Federal Census on microfilm for every county within the local state and perhaps neighbouring states. Obviously, if someone lives in Arizona, they might have difficulty finding census records for Michigan. Genealogical newsgroups are a good resource when you live far from the state where you would like to do research. Post a message offering to trade census look-ups in your state with someone in the remote state.
Chances are someone in Michigan has a similar problem of needing information from Arizona.
Soundex is a mathematical function which converts a surname into a code usually consisting of a letter followed by 3 digits. These index cards have been microfilmed for ease in searching. The Soundex guide will locate the actual page and line number where your relative's record can be found so that you don't have to stare at hundreds or thousands of microfilmed pages. The information contained in the census records changed nearly every decade. From 1790-1840 only the head of household is listed by name.