Literally, this term means habit, practice, customary procedure, action, norm, or usage sanctioned by tradition. More specifically, Sunnah refers to Prophet Muhammad's sayings, practices, and habits. The Hadith of the Prophet constitute a written record of his Sunnah.
In general, the word Sunnah means habit, practice, customary procedure, or action, norm and usage sanctioned by tradition. In specific, any time the word Sunnah is mentioned, it is to refer to Prophet Muhammad (s.a.w.). Here it means his sayings, practices, living habits. The Hadith are reports on the Sunnah.
The two major legal sources of jurisprudence in Islam are the Quran and the Sunnah.
The Sunnah may confirm what is mentioned in Qur'an, interpret and explain it, specify what is meant by some general verses, limit and restrict the meaning of some verse in it, or may explain something that has been revealed in Qur'an.
The Sunnah has a high authority in Islam; and Allah in many places in the Qur'an orders the Muslims to follow the teachings of Prophet Mohammad (s.a.w.).
All the traditions and practices of Prophet Muhammad (S.A.W.), that are recorded not only in such books as Sahih Bukhari and Sahih Muslim, but also in living people to whom these traditions and practices have been transmitted, from person to person, from then until now. Although the Sunnah refers almost exclusively to the practice of the Messenger of Allah, it also comprises the customs of the first generation of Muslims in Madinah. They learned their Islam directly from the Prophet, not from books, and transmitted what they had learned to the next generation. Thus they have become models whom all Muslims should follow. The Sunnah is a complete behaviourial science that has been systematically kept outside the learning framework of this society. It cannot be learned from books, although books can be helpful. It can only be learned from real Muslims who are true teachers. Plural: Sunnan.