The questions asked on each census vary from decade to decade.  The first census in 1790 looked at three different variables, while the 2000 census looked at over 70 different variables.  Because the questions are not consistent from census to census, though, it is often difficult to compare changes from decade to decade. 

If you want to compare different variables, you will want to make sure the censuses you are comparing have used the same variable.  You can find a chart compiled by the Documents Center at the University of Michigan that lists all the variables that have been examined by each Census of Population and Housing.  Suzanne Schulze also has written a number of books that allow you to see what variables are included in any given census volumes and at what level of geography.