The U.S. flag is a strong symbol of American identity and national pride. Stars are a symbol of the heavens and the goals to which humankind aspires; stripes are symbolic of rays of light from the sun. 13 stripes represent the original thirteen colonies that declared independence from England; fifty stars symbolize the current 50 United States (see dates of statehood for all 50 states).
"Old Glory" has had a colorful history, and there are so many excellent web pages on the American flag that we cannot hope to improve on the presentation; a great place to begin is the Smithsonian's Star Spangled Banner website.
The American flag is the focus of the Marine Corp Memorial in Washington D.C. (photo by dbking on Flickr; use permitted with attribution). See All State Flags.