Censuses as a procedure are systematic way of acquiring and recording socio-demographic information regarding the population in a nation. The essential features of a population is collected every ten years. Census is opposite to sampling.
For the United States of America, census is particularly important for political reasons, because legislative seats and seats in the House of Representatives are given in accordance with the population of the states. For example, if population of the states grows in a period of ten years, that state should get more seats in the House of Representatives. At the same time, some other state would lose it seats. The number of seats defines a justifiable representativeness in US politics.