Aaron Copland (1900 – 1990) was an American composer, composition teacher, writer, and later in his career a conductor of his own and other American music. He was instrumental in forging a distinctly American style of composition. He is best known to the public for the works he wrote in the 1930s and 1940s in a deliberately more accessible style than his earlier pieces, including the ballets Appalachian Spring, Billy the Kid, Rodeo and his Fanfare for the Common Man.

The open, slowly changing harmonies of many of his works are archetypical of what many people consider to be the sound of American music, evoking the vast American landscape and pioneer spirit. However, he wrote music in different styles at different periods of his life: his early works incorporated jazz or avant-garde elements whereas his later music incorporated serial techniques. In addition to his ballets and orchestral works he produced music in many other genres including chamber music, vocal works, opera and film scores.

http://en.wikipedia.org/wiki/Aaron_Copland