Census Data
A census has been undertaken every 10-years since 1841, but not every census provides detailed data. The Census is also just a snapshot of what was going on on the day of Census.
Early census date just showed numbers of people living in roads or houses, Later details available may include: address, full names of all occupants on that day (whether permanent residents or just visiting), occupations, ages, relationship to other occupants and marital status.
If you can't find a male ancestor in the 1861 census then it could be because they were a serving soldier. For that information you would need to consult The 1861 Worldwide Army Index with the names of 245,000 men stationed in Britain and around the British Empire.
The census returns for England & Wales 1841-1911 and the census returns for Scotland 1841-1901 are incomplete and have pieces missing. The 1841 census is missing parishes from eleven counties in England, but the 1891 census is only missing some house numbers.
The first to record the details of individuals was the 1841 census, although it contains less information than the censuses that followed. The 1841 census enumerators were instructed to round down a person’s age to the nearest multiple of five. A person aged 69, for example, was enumerated as 65. Due to the personal nature of census information, a 100-year secrecy rule is in place. This means the most recent census available is the 1921 census.
The 1931 Census was largely destroyed by fire and a census was not conducted in 1941 because of the Second World War. The next census data will therefore no be available until 2051 for the 1951 Census.
A fuller description of the UK Census history can be found on Wikipedia here.