The United States is called a melting pot, because it a merge of cultures, ideas, and peoples.
A more immigrants come, the United States will eventually cease to have a White, generally Anglo-Saxon majority. This means that the minorities will have the majority over the white people. This should happen in the middle of the 21st century, when non-Hispanic whites will have < 50% of the US population.
Until recently, the largest immigrant group has been the African American population. In 2000, the Hispanic population outnumbered the African American population. 1/8 of all Americans are descendants of Africans brought to the US to become slaves. Because of years of racism and discrimination, 24% of African Americans lived below the poverty line, while only 10% of white people lived below the poverty line.
Recently, they've been getting more and more political power. The number of African American elected officials is 7 times larger today than in 1970. For example, in 2001, Colin Powell was named Secretary of State, and in 2005, Condoleezza Rice became the 2nd black Secretary of State.
Asian Americans have been called the super-achievers of the minority majority. 49% of Asian Americans older than 25 have a college degree, twice the national average.
Despite the many races and cultures that make up America, most Americans share a common political culture, an overall set of values widely shared within a society. For example, most Americans believe that everyone should be treated equally.