The national flag 

The U.S. flag has undergone many changes since the first official flag of 1777. On June 14, 1777 , the Continental Congress passed the first Flag Act, which said that the flag would be made up of thirteen alternating red and white stripes and thirteen white stars on a blue field. Stars have been added to the flag as new states join the union. Currently, the flag contains 50 stars.

White: Signifies purity and innocence

Red: Signifies valor and bravery

Blue: Signifies Vigilance, perseverance, and justice

There are 50 stars representing the 50 states and there are 13 stripes representing the 13 original states.

Flag: thirteen equal horizontal stripes of red (top and bottom) alternating with white; there is a blue rectangle in the upper hoist-side corner bearing 50 small white five-pointed stars arranged in nine offset horizontal rows of six stars (top and bottom) alternating with rows of five stars; the 50 stars represent the 50 states, the 13 stripes represent the 13 original colonies; known as Old Glory; the design and colors have been the basis for a number of other flags including Chile, Liberia, Malaysia, and Puerto Rico.

Reference from www.usconsulate.org.hk/pas/kids/americans1.htm