A census is the official record of where people lived on a certain date. Since 1801, censuses have been taken in Britain every ten years, with the exception of 1941 (due to WW2).
Until 1851, censuses were just taken as head counts with very limited individual detail recorded. The schedules were sent out to each household and the head of that household was required to record who was resident at midnight on a given date. The enumerators' books were collected together in larger areas known as Registration Districts. These cover the same area as Civil Registration Districts for Births, Marriages and Deaths.
National Archives/Public Record and County Record Offices allow access to census returns and you can purchase copies or transcriptions of census records from Parish Chest.