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.