На википедии все доходчиво описано:http://ru.wikipedia.org/wiki/%D0%9C%D0%B5%D0%B6%D0%B4%D1%83%D0%BD%D0%B0%D1%80%D0%BE%D0%B4%D0%BD%D1%8B%D0%B9_%D1%83%D0%B3%D0%BE%D0%BB%D0%BE%D0%B2%D0%BD%D1%8B%D0%B9_%D1%81%D1%83%D0%B4