When writing the census returns the enumerators used some abbreviations for occupations and status, especially for the 1841 census although these gradually disappeared over the following census returns and the descriptions became clearer.

Ag. Lab. - Agricultural Labourer
Annuitant - Someone on an annual allowance or a pensioner in an institution
Ap - Apprentice
Army - Any member of HM land forces regardless of rank
Boarder - A person living with a family
Cl - Clerk
Dressmaker - This profession was also often given by prostitutes
F.S. - Female Servant
H.P. - Members of HM armed forces on half-pay
Idiot - Someone with congenital mental deficiency
Ind. - Independent - people living on their own means
In-Law - As in Brother-in-law & Sister-in-law but could be used for a 'step' relative
J - Journeyman, someone who has completed their apprenticeship but not yet become a master
Lodger - Someone with separate accommodation to the family
Lunatic - Someone with mental illness
M - Manufacturer
m - Maker, as in shoe maker
Mar or M - Married (in column marked condition as in marriage)
M.S. - Male Servant
Navy - Any member of HM naval forces, including marines, regardless of rank
P - Pensioner in HM armed forces
Rail. Lab. - Railway Labourer
Scholar - A child over 5 receiving daily tuition
Serv - Servant
Sh - Shopman
S - Single (in column marked condition as in marriage)
Unm or U - Unmarried (in column marked condition as in marriage)
W - Widow or Widower (in column marked condition as in marriage)

A United Kingdom census has been taken every 10 years since 1801 except 1941 during the second world war. The census returns for 1801, 1811, 1821 and 1831 were not preserved but a few have survived in some areas.
The date each census was actually taken was as follows:-

1841 - 7th June 1841
1851 - 30th March 1851
1861 - 7th April 1861
1871 - 2nd April 1871
1881 - 3rd April 1881
1891 - 5th April 1891
1901 - 31st March 1901
1911 - 2nd April 1911

The 100 year closure rule was established after 1911 so subsequent censuses will not be released until their 100 years are up.