The U.S. Flag consists of 13 red and white stripes with a blue rectangle in the canton, or top left, containg 50 stars.
The flag was first standardized by Congress in 1777 with the passage of the first Flag Act. It called for our official standard to be made with thirteen stripes, alternating red and white along with thirteen white stars in a blue field.
The arrangement of these stars had changed as the nation grew, but the symbolism remains the same. Each star represents one of the states in the U.S. with the current number equally fifty. The blue color represents vigilance, perseverance and justice.
The stripes of red and white number thirteen in recognition of the thirteen original colonies. The color white represents purity and innocence while the red represents hardiness and valor.
It is important that these explanations were never made specifically about the flag, rather they were made about the great seal which shares the flags colors.