Every 10 years, as required by the US Constitution, a count is made of all the people who live in the United States. 

In March of 2010, census forms will be delivered to every residence in the United States and Puerto Rico and we'll be asked to answer the 10 questions on the form and mail it back to the Census Bureau.

An accurate count of the people living in the US has a direct effect on how federal financial resources are allocated to each community. 

It also affects the re-districting of political boundaries to reflect any increases in population.  For example, Arizona may be eligible for two more Congressional seats and some counties may require additional supervisors to serve their larger populations.

Learn more.  These websites will help you do just that ...

City of Phoenix
US Census Bureau
MAG - Maricopa Association of Governments