Obligatory prayers are the prayers which Almighty Allah has commanded us to offer five-times a day at their fixed times. They are Fajr (Dawn) prayer, Zhuhr (Noon) prayer, `Asr (Afternoon) prayer, Maghrib (Sunset) prayer, and `Isha’ (Night) prayer. Abu Hurairah (may Allah be pleased with him) narrated that the Prophet (peace be upon him) said, “The five daily (prescribed) Salat, and Friday (prayer) to the next Friday (prayer), and the fasting of Ramadan to the next Ramadan, is expiation of the sins committed in between them, so long as major sins are avoided.” (Muslim)
Prophet Muhammad (peace be upon him) offered extra voluntarily prayers in addition to obligatory prayers. These prayers are called Sunnah or Nafl prayers. Supererogatory prayers are divided into two categories: first is the regular sunnah prayers (rawatib) which are mentioned in the following hadith:
Um Habibah (may Allah be pleased with her narrated that the Prophet (peace be upon him) said: “Whoever prays twelve rak`ahs during the night and day other than the prescribed prayers, a house will be built for him in Paradise: four before Zhuhr and two rak`ahs afterward, two before `Asr, two after Maghrib and two before Fajr.” (At-Tirmidhi)
Second, there are many supererogatory prayers aside from these regular sunnah prayers such as two rak`as after ablution or after entering a mosque.
The supererogatory prayers are normally offered individually without Adhan (call to prayer) or Iqamah.