Sunnah prayers (salat al-sunnah) are voluntary prayers that Muslims perform in addition to the five obligatory daily prayers, following the practice of the Prophet Muhammad. They are recommended rather than required, and they use the same steps as the obligatory prayers.
The best-known set, called the confirmed sunnah or al-sunan al-rawatib, totals 12 rak’ahs (prayer units) a day: 2 before Fajr (dawn), 4 before and 2 after Dhuhr (midday), 2 after Maghrib (sunset), and 2 after Isha (night). Other voluntary prayers, such as 4 rak’ahs before Asr, Witr at night, and Duha in the late morning, are also recommended, and details vary somewhat between schools of Islamic law.
Source: IslamQA: Sunnah Prayers (Al-Sunan Al-Rawatib)
Image: Muslim Kid Praying, by chidioc, CC0, via Wikimedia Commons.