Census An official count and description of the people living in a country, colony, state, county, township, or city (FamilySearch Wiki).
In most countries census records were taken every few years. In the US and British Colonies they happened every 10 years. Some states even took censuses in the years in between. The purpose was to help with taxation, population count and governing. What is great about them for genealogists, is that most of them named (or at least referenced) everyone in the household. More recent ones even included relationships which can help put families together. They give you a snapshot of the family at that time. You can often find great pearls of information such as occupations, deceased children, extended family, immigration details and marriage info. They are the perfect launching point for all genealogy.