The tarawih prayer is comprised of twenty rak'as, prayed in pairs.
It is prayed on each of the nights of Ramadan.
In each pair of rak'as, one intends to pray tarawih or the qiyam of Ramadan.
One starts each pair with the opening du'a.
It is not valid to perform every four rak'as together finishing with one salam.
The time of tarawih extends from after Isha till the break of dawn- the same as witr.
It is recommended to perform it before witr, and to read a complete chapter of Quran in it each day.
Furthermore, a congregation is recommended for both tarawih and witr.
The evidence that tarawih prayer, and praying it in congregation, are sunna is what Aisha (G. p. H) narrated: "The Messenger of Allah (P. B. U. H) went out in the middle of the night and prayed in the mosque and some men prayed behind him.
The next morning, people spoke of this and (that night) a large number of them gathered and prayed behind him.
The next morning the people spoke again of this and again that night the mosque was filled with a large number of people. (pa: 205)
The Messenger of Allah (G. p. H) came out (to the mosque) and the people prayed behind him.
On the fourth night the mosque was overwhelmed with people and could not accommodate them, but the Prophet (P. B. U. H) came out (only) for Fajr.
When he finished Fajr, he turned towards the people, recited tashahud, and said: 'Your presence was not unknown to me but I was afraid lest tarawih be enjoined on you and you might not be able to keep it up. ' So, Allah's Messenger (P. B. U. H) died and the situation remained like that (people praying individually)".
[Al-Bukhari, Vol. 2, The Book of Tarawih Prayer, Chapter 1/1908].
The situation remained as such during the caliphate of Abu Bakr and first part of the caliphate of Umar (G. p. H).
Umar (G. p. H) then gathered the men to pray tarawih and appointed Abu Ka'eb (G. p. H) as their imam
[See Al-Bukhari, Vol. 2, The Book of Tarawih Prayer, Chapter 1/1906].
He also gathered the women and appointed Sulaiman ibn Abu Huthma, as their imam.
Uthman (G. p. H) said during his caliphate, referring to Umar's action:
"May Allah (G. B. H) enlighten the grave of Umar as he has enlightened our mosques".
The evidence that tarawih is twenty rak'as is what Al-Sa'eb ibn Yazeed Al-Sahabi (G. p. H) said: "They used to perform twenty rak'as in qiyam in the month of Ramadan, at the time of Umar ibn Al-Khattab (G. p. H).
He added: They used to read those suras whose number of verses is greater than hundreds of verses, and used to lean on their sticks at the time of Uthman ibn Affan (G. p. H) due to long standing".
[Al-Bayhaqi, Vol. 2, Page 496].