Sunnah (Arabic: سُنَّة‎, sunnah, plural Arabic: سُنَن‎ sunan [sunan]), also sunna or sunnat, is the body of literature which discusses and prescribes the traditional customs and practices of the Islamic community, both social and legal. According to classical Islamic theories of Sunna, Sunna is made up of the customs and practices of the Prophet Muhammad, (Peace be Upon Him) documented by hadith (the verbally transmitted record of the teachings, deeds and sayings, silent permissions or disapprovals of Prophet Muhammad (Peace be Upon Him), which along with the Quran (the holy book of Islam), are the divine revelation (Wahy) delivered through the Prophet that make up the primary sources of Islamic law and belief/theology.
According to Muslim belief, Prophet Muhammad (Peace be Upon Him) was the best exemplar for Muslims, and several verses in the Quran declare his conduct exemplary, and enjoin his followers to obey him. Sunnah provides a basis not only for how to pray and major laws in Islam, but for "even the most mundane activities", such as the order in which to cut fingernails or the proper length of a beard.

In addition to being defined as "all the traditions and practices" of the Prophet that "have become models to be followed" by Muslims, sunnah is defined in Arabic as "a path, a way, a manner of life". Recording the sunnahs is also an Arabian tradition and once they converted to Islam, the Arabs brought this custom to their religion.  The classical meaning of Sunnah was introduced in the late second century of Islam, when under the influence of the scholar Al-Shafi‘i, Prophet Muhammad's (Peace be Upon Him) example as recorded in hadith was given priority of over all other precedents set by other authorities. The term al-sunnah then eventually came to be viewed as synonymous with the sunnah of Prophet Muhammad (Peace be Upon Him) based on hadith reports.

Abū ʿAbdullāh Muhammad ibn Idrīs al-Shāfiʿī (150-204 AH), known as Imam Al-Shafi'i, argued against flexible sunnah and the use of precedents from multiple sources, emphasizing the final authority of a hadith of Prophet Muhammad (Peace be Upon Him), so that even the Qur'an was "to be interpreted in the light of traditions (i.e. hadith), and not vice versa." While the sunnah has often been called "second to the Quran", hadith has also been said to "rule over and interpret the Quran". Al-Shafiʿi "forcefully argued" that the sunnah stands "on equal footing with the Quran", both being divine revelation. As Al-Shafi'i put it, “the command of the Prophet is the command of God."  Sunnah of Prophet Muhammad (Peace be Upon Him) outranked all others, and "broad agreement" developed that "hadith must be the basis for authentication of any Sunnah,"  Al-Shafiʿi's success was such that later writers “hardly ever thought of sunnah as comprising anything but that of the Prophet”, and sunnah was often considered synonymous with hadith.

Classical Islamist  jurists divide sunnah into that which has no legal consequences --al-sunna al-ʿādīyah -- the "personal habits and preferences" of Prophet Muhammad (Peace be Upon Him); and that which is binding on Muslims -- al-sunna al-hudā. Following  non-binding al-sunna al-ʿādīyah is meritorious but not obligatory. Sufis see the "division between binding and non-binding" sunnah as "meaningless". Prophet Muhammad (Peace be UponHim) is al-insān al-kāmil, the perfect man, Habib-Allah beloved of God, an intercessor, a "channel of divine light". Imitating his every action is "the ultimate expression" of piety. or in the words of Imam Al-Ghazālī: "Know that the key to joy is following the sunnah and imitating the Prophet in all his comings and goings, words and deeds, extending to his manner of eating, rising, sleeping and speaking. I do not say this only in relation to requirements of religion [ibādāt], for there is no escaping these; rather, this includes every area of behavior. 

Sunnah Salat:
Salât as-Sunnah (Arabic: صلاة السنة) are optional prayers performed in addition to the five daily compulsory Salât prayers. Some are done at the same time as the compulsory prayers, some are done only at certain times, e.g. late at night, and some are only done for specific occasions such as during a drought. They are called sunnah because how they are practiced as based on stories, narrations, interpretations, traditions of Prophet Muhammad (Peace be Upon Him) by his companions. "Examples include al-Sunan al-Rawaatib" (sunnah prayers which Muhammad did regularly), "Salaat al-Duhaa and so on." Sunnah Mu’akkadah are actions Prophet Muhammad (Peace be Upon Him) "never omitted to do, whether he was travelling or not," such as the prayers Sunnat al-Fajr and al-Witr.

Types of sunnah:

Sunnah upon which fiqh is based may be divided into:

Sunnah Qawliyyah - the sayings of Prophet Muhammad (Peace be Upon Him), generally synonymous with “hadith”, since the sayings of Prophet Muhammad (Peace be Upon Him) are noted down by the companions and called “hadith”.

Sunnah Fiiliyyah - the actions of Prophet Muhammad (Peace be Upon Him), including both religious and worldly actions.

Sunnah Taqririyyah - the approvals of the Prophet regarding the actions of the Companions which occurred in two different ways:
When Prophet Muhammad (Peace be Upon Him) kept silent for an action and did not oppose it.
When the Prophet showed his pleasure and smiled for a companion's action.