Вот тут неплохо написано:http://www.wikireality.ru/wiki/%D0%9F%D0%B5%D1%81%D0%B8%D0%BA%D0%BE%D1%82#.D0.9E.D0.B1.D0.B2.D0.B8.D0.BD.D0.B5.D0.BD.D0.B8.D1.8F_.D0.B2_.D1.84.D0.B8.D0.BD.D0.B0.D0.BD.D1.81.D0.BE.D0.B2.D0.BE.D0.BC_.D0.BC.D0.BE.D1.88.D0.B5.D0.BD.D0.BD.D0.B8.D1.87.D0.B5.D1.81.D1.82.D0.B2.D0.B5