The United States of America ( ) is one of the fifty states and one federal district. Constitutional federal republic, east to the Atlantic, West Pacific, north of Canada, south of Mexico. The United States of America is a multicultural and multiracial country; land area more than 9620000square kilometers, ranking third in the world or in fourth; the total population of over three hundred million people, ranking third in the world. On July 4, 1776, the Continental Congress in Philadelphia formally adopted the" Declaration of independence", declaring the United States was born. Since 1870the United States national economy at first in the world, UN Security Council as one of five permanent members, today's gross domestic product is more than20% of the world. In the economic, political, scientific, military, entertainment and many other fields have led the huge influence of global, is currently the world's only superpower.