Sunnah has several meanings, including path or an act regularly observed by Prophet Muhammad (s.'a.s.), such as observation of two raka'aats before morning prayer, and fasting three days in every month.
The concept of Sunnah is better understood when contrasted with the concept of Fard. Fard is an imperative duty - and the failure to observe it is a sin punishable by Allah.
Sunnah, on the other hand, is optional; thus the Sunnah duties are non-imperative and failure to observe them is not punishable by Allah. However, Sunnah is equally important because we are constantly enjoined by the Quran and Hadeeth to observe Sunnah, and frequent negligence of Sunnah Prayers is considered to be an act of sin.
Each of the five daily prayers has Sunnahs to be observed, either before or after the Fard prayer. They are as follows:
1. SUNNAH OF SALAATUS SUBH
Salaatus Subh, morning prayer, has two raka'aats of Sunnah prayer (they are known as raka'atail Fajr) before the morning prayer. There is no Suunah until the sun is about seven feet high.
According to the Prophet's wife, 'Aaishaa, the Prophet said:
"The two raka'aats at dawn are better than this world and what it contains." (Muslim)
The Prophet also has been reported as saying that he used to recite in these two raka'aats chapter Al-Kaafiruun, and Al-Ikhlaas. Abu Hurairah reported that the Prophet said "Whoever misses two raka'aats of Fajr, so that the sun has risen, should pray them." (Baihaqee)
2. SUNNAH OF SALAATUZ ZUHR
Salaatuz Zuhr has two or four raka'aats of Sunnah before the Fard and two or four after. In Hadeeth by Ibn 'Umar:
"I learned from the Prophet ten raka'aats: two before Zuhr, and two after Zuhr; two raka'aats after Maghrib in his house, two raka'aats after 'Ishaa in his house and two raka'aats after Subh prayer." (Al-Bukhaaree)
Abu Ayyuub Al-Ansaaree was asked why did he constantly offer four raka'aats before Zuhr; he replied: I saw the Prophet do it and when I asked him he said:
"It is a time when the doors of heaven are opened and I hope that a good deed would be risen for me in that time." (Ahmad)
The Prophet's wife, 'Aaishaa, reported that the Prophet used to make up the missed Sunnahs of Zuhr.
3. SUNNAH OF SALAATUL 'ASR
'Asr prayer has two or four raka'aats Sunnah before the Fard prayer. That is in accordance with the Hadeeth reported by Ibn 'Umar, in which the Prophet said:
"May Allah's mercy be upon a person who offered four raka'aats (of Sunnah) before 'Asr." (Tirmidhee)
However, there is no Sunnah after 'Asr prayer.
4. SUNNAH OF SALAATUL MAGHRIB
Maghrib prayer has two raka'aats of Sunnah after the Fard prayer; there is no Sunnah before Maghrib. The Sunnah in this prayer is to recite chapters Al-Kaafiruun and Al-Ikhlaas, and to offer them in the house.
5. SUNNAH OF SALAATUL 'ISHAA
Salaatul 'Ishaa has two or four raka'aats of Sunnah before Fard, and two raka'aats of Sunnah after. There is also Salaatul Witr after 'Ishaa.