In Islam, the Arabic word sunnah has come to denote the way Prophet Muhammad (Peace Be Upon Him)), the Messenger of Allah, lived his life. The Sunnah is the second source of Islamic jurisprudence, the first being the Qur'an. Both sources are indispensable; one cannot practice Islam without consulting both of them. The Arabic word hadith (pl. ahadith) is very similar to Sunnah, but not identical. A hadith is a narration about the life of the Prophet (Peace Be Upon Him)) or what he approved - as opposed to his life itself, which is the Sunnah as already mentioned.
Eventhough Islam is split into different groups, they all basically follow the teachings of Koran.