Sunnah (Arabic) in Islam is both the life of Muhammad (the "sira") and his recorded and validated sayings (the "hadith"). These are together supposed to constitute a moral example for the Muslim to study, discuss, and emulate.
There is a tradition both of historical biography ("Ilm ar-Rijal") of Muhammad and of validating hadith - isnah or "backing". These were influential in the development of early Muslim philosophy and modern scientific citation.
Sunnah must be distincted from both fiqh, which are opinions of the classical jurists, and the Qur'an, which is revelation, not record. It is one of many terms in Islam which are difficult to translate out of Arabic without loss of meaning. History further complicates the translation since different assumptions about sunnah dominated Islam in past eras:
In early Islam, the Sunnah was often considered equivalent to the sira, as the hadith were poorly validated, and contemporary commentators on the Prophet's life were better known. As the hadith came to be better documented, and the scholars who validated them gained in prestige, the sunnah came to be known mostly through the hadith, especially as variant or fictional biographies of Muhammad spread, in part from the Christian world, some of them very slanderous. Classical Islam often equates the sunnah with the hadith.
Modern Islam is beginning to examine both the sira and the hadith, with an eye to justifying modifications to the fiqh, or jurisprudence, which was largely drawn from past interpretations of both. The sunnah in one form or another would retain its central role in providing both a moral example (sira) and ethical guidance via the Prophet's own social rules (hadith).