All the prayers, other than Fardh and Sunnah, are Nafl prayers. There is no limit to them and the more Nafl one offers the greater will be the credit and reward for them. There have been such servants of Allah who used to offer Nafl prayers throughout the night and did not sleep.
Of all the Nafl prayers some have a very great credit. Therefore it is better to offer those instead of others because a little amount of extra labour entitles one for great reward. They are:
(i) Tahiyatul Wudu, (ii) Tahiyatul Masjid, (iii) Ishraq, (iv) Chasht, (v) Awwabin, (vi) Tahajjud and (vii) Salatul-Tasbih.
(i)-Tahiyatul Wudu is of two Raka’ats and is offered whenever ablution is performed. Its virtue has been greatly mentioned in the Traditions.
(ii)-Tahiyatul Masjid is of two Raka’ats and may be offered on entering the mosque before sitting.
(iii)-Ishraq: After offering the Fajr prayer one should remain sitting on the prayer -mat till the sun rises and is sufficiently high: but during this period one should remain reciting Darood Sharif or Kalima or remembering Allah or any other prayer (du’a) and should not indulge in useless worldly talks or business. When the sun is high, offer two or four Raka’ats. Its reward is equal to the reward of a Hajj or Umrah. But if one got engaged in worldly affairs after Fajr prayer and then offered Ishraq prayer; then it be proper, but its reward will be decreased.
(iv)-Chasht: When the sun is sufficiently high and hot, two or four Raka’ats are offered as Chasht prayer. Its reward is also very great.
(v)-Awwabin: After the Fard and Sunna prayers of Maghrib, six to twenty Raka’ats are offered as Awwabin prayer.
(vi)-Tahajjud: Offering of prayer on rising after mid-night, is of great merit and virtue. This called Tahajjud prayer. This is most acceptable to Allah and its credit and reward is the greatest. At least four and twelve at the most are its Raka’ats. If not four, only two may also be offered. If one cannot be sure to get up after mid-night, then he may offer it after Isha prayer; but its reward will not be to that extent. In addition to these as many Nafl prayer may be offered as one wishes.
(vii)-Salatul-Tasbih: Great merit and reward has been attributed to this prayer in the Traditions. This was taught by the Holy Prophet (Sallallahu Alayhi Wasallam) to his uncle Hadrat Abbas (radhiallahu ta’ala anho) and told that on account of offering this prayer all of his past and future, old and new, great and small sins will be forgiven by Allah and said that if possible this should be offered daily, once a week, or once a month, or once a year and even if this is not possible, then once in the whole life.
[from Maulana Ashraf Ali Thanvi’s famous “Bahishti Zewar”, p. 115-116]