How does the government count the population of a place?

Many national governments of the world perform a regular count of their populations - in English we call it a 'census', and it is stated in the Constitution of the US that this should be done periodically (we do it every 10 years). The results are used by local, state and federal governments for many purposes, including: determining a tax base (how much tax can be expected for an area or region), and to determine how much representation an area will have in governmental bodies (for example, the number of State Representatives each US state will have in the House of Representatives). Census results help administrators to plan appropriately for future expenses, where to build hospitals and schools, even where to put highways and traffic lights!