Sunnah (سنة, plural سنن sunan, Arabic) is derived from the root (سن Arabic) meaning smooth and easy flow or direct flow path. The word literally means a clear and well trodden path. In the discussion of the sources of religion, Sunnah denotes the practice of Prophet Muhammad that he taught and practically instituted as a teacher of the sharī‘ah and the best exemplar. According to Muslim belief, this practice is to be adhered to in fulfilling the divine injunctions, carrying out religious rites and moulding life in accord with the will of God. To institute these practices was, the Qur’ān states, a part of the Prophet’s responsibility as a Messenger of God

The sunnah of Muhammad includes his specific words, habits, practices, and silent approvals: it is significant because it addresses ways of life dealing with friends, family and government. Recording sunnah was an Arabian tradition and, once people converted to Islam, they brought this custom to their religion. The sunnah is consulted after referring to the Qur'an, if the issue is not addressed there. The term "Sunni" denotes those who claim to practice these usages, as part of the Ummah.

Read more about Sunnah:  Etymology, Sunnah and Hadith, Sunnah and Islam, Alternative Views On Sunnah