Census records are invaluable for genealogists. We all know to start with the most recent census for our area of interest, and work backwards.
Nationwide census records in the USA were taken every 10 years, on the decade (1930, 1920, 1910 etc)
Census records in England and Canada were taken every 10 years but in the first year after the decade (1911 which is not public yet for UK, 1901, 1891 etc)
I think most of us know that from a census record we can figure out family groups, get approximate years of birth and birth locations. But what else do census records tell us?
Of course it depends on the year of the record and the country it was taken in. But let's talk generalities.
When you consult a census page you should always check names of neighbours. Often a neighbour is a father or a brother or a married sister (or some other relative). Be sure to copy the entire page so you can refer to it later.
Check the occupation of each individual. Often that provides clues for other research. If a man's occupation is given as farmer, you will want to hunt for land records. If he is a skilled worker (blacksmith for example) you may want to check City Directories to see if he is listed. Perhaps his place of employment is given in a more recent census. Now you can hunt for details on that workplace; perhaps they even have records you can consult.
What about religion? In Canadian census records Religion is provided. This is very helpful in finding church records to hunt for baptisms, marriages or burials.
Check to see birth locations for each of the children. Not only does this allow us to seek birth records (whether church records or vital registrations), we can also learn a family's migration patterns. Best of all we can often narrow an immigration year if the family includes an ancestor who left one country to go to another. If child A was born in the old country and child B was born in the current country a few years later, you've got a nice time frame to hunt for ships passenger lists
Census records may also provide an approximate year of immigration or of naturalization. I use the word "approximate" because those two years are often mis-remembered. You need to search a year or two on either side of any year given. Just as one example, the 1900, 1910 ,1920 and 1930 American census records identify citizenship status, with notations showing the individual was an Alien, or had started the Naturalization process or had his final papers. Some early (1841/1842) census records for Canada give a year of immigration if the individual lived at any time outside of Canada. Later census records also provide a year of immigration for individuals not born in Canada.
Marriage years are also found on some census records. Some census ask for a year of marriage, some ask how old each married person was when they first married -- it varies so be aware and read the questions on each census carefully.
Questions asked on Canadian Census are given here in full for each available census year. You might be surprised at the wealth of details you can find!
Blank Census Forms for USA, Canada and UK are found online and are free