Abu Hurairah RAA reported that the Messenger of Allaah (sallallaahu alayhi wasallam) said:
"Whosoever performs the night prayer on the
night of Al-Qadr with Eemaan (firm belief) and seeking reward will have all his
past sins forgiven ." 
This hadeeth is evidence for the virtue of the night of Al-Qadr as well as performing qiyaam during it. And it indicates that it is a grand night, which Allaah has honored and made better than a thousand months in regards to its blessing and the blessings found in the righteous deeds that are performed in it. Thus it is better than the worship of a thousand months and that is equivalent to eighty-three years and four months. Due to this, whoever performs qiyaam (night prayer) with true faith and while seeking reward in it, will be forgiven his past sins. There were certain verses verses concerning this virtue:
"We sent it (the Qur'aan) down on a blessed
night. Verily We are Ever-Warning. Therein (on that night) is decreed every
matter of ordainment ." 
Therefore, it is a “blessed night” meaning it possesses much good and blessing due to its merit and the great reward that awaits the one who does good deeds in it. Among its blessings, is that Allaah revealed the Qur’aan in it. Allaah says:
"Verily, We have sent it (the Qur'aan) down in
the night of Al-Qadr. And what will make you know what the night of Al-Qadr is?
The night of Al-Qadr is better than a thousand months. Therein descend the
angels and the Rooh (Jibreel) by their Lord's permission with all decrees. (All
that night) there is peace, until the appearance of dawn ." 
Ibn Katheer (ra) said concerning Allaah’s saying: “Therein descend the angels and the Rooh”:
“This means that the descending of the angels increases during this night due to the vast amount of its blessings. And the angels descend along with the descent of blessing and mercy, just as they descend during the time when the Qur’aan is recited and encompass the gatherings in which Allaah is remembered and spread their wings for the true seeker of knowledge, showing respect for him.”
This night occurs only in Ramadaan, since Allaah revealed the Qur’aan in it. He informs us that its revelation occurred during the month of Ramadaan in His saying: "Verily, We have sent it (the Qur'aan) down in the night of Al-Qadr." 
And His saying: "The month of Ramadaan in which
was revealed the Qur'aan " 
This means that its revelation from Allaah to His Prophet Muhammad (sallallaahu alayhi wasallam) began in it.
Allaah’s statement: “The night of Al-Qadr” is either an indicative of the honor and station of that night, as it is said: "Such and such person has great Qadr". The conjunction of "the night" to Al-Qadr is the joining of a descriptive feature to it, thus making it mean "An honorable night". The word “Al-Qadr” may also be in reference to the ordainment and disposal of affairs. Thus, its being joined to the word "the night" would be in order to denote a place or time to it. So it would mean "the night in which all that will occur in the following year will be decreed." This is similar to Allaah's saying: "Therein (on that night) is decreed every matter of ordainment." 
Qataadah said of this: "Therein is decreed every
matter of the (upcoming) year" and Ibn Al-Qayyim said that this is the
correct opinion . 
What seems most correct is that there is nothing that restricts the possibility of these two understandings and Allaah knows best.
His statement: “with Eemaan” means with firm conviction in what Allaah has prepared for those who stand in prayer during this magnificent night. And “seeking reward” means looking for reward and the attainment of recompense.
Thus, this is a grand night, which Allaah has chosen for beginning the revelation of the Qur’aan. So the Muslim must acknowledge its weight in worth, by guarding it and spending it in worship whilst having firm conviction and seeking the reward of Allaah, in order that Allaah may forgive all of his previous sins. This is why the Prophet (sallallaahu alayhi wasallam) warned us about being heedless of this night and being neglectful of spending it in worship, for the Muslim would be prevented from its good.
Abu Hurairah reported that the Messenger of Allaah (sallallaahu alayhi wasallam) said:
"Ramadaan has come to you -- a blessed month. Allaah has made it obligatory upon you to fast in it. During this month, the gates of heaven are open, the gates of the Hellfire are closed and the evil devils are chained. To Allaah belongs a night in it, which is better than a thousand months. Whosoever is prevented from its good, then he has been prevented."