Volume 3, Book 41, Number 593:
Narrated ‘Abdullah (radiallaahu `anhu):
I heard a man reciting a verse (of the Holy Qur’an) but I had heard the Prophet (sallallaahu `alayhi wasallam) reciting it differently. So, I caught hold of the man by the hand and took him to Allah’s Apostle (sallallaahu `alayhi wasallam) who said, “Both of you are right.” Shu’ba, the sub-narrator said, “I think he said to them, “Don’t differ, for the nations before you differed and perished (because of their differences). “
The Qur’aan was revealed in one style at the beginning, but the Messenger of Allaah (peace and blessings of Allaah be upon him) kept asking Jibreel until he taught him seven styles, all of which were complete. The evidence for that is the hadeeth of Ibn ‘Abbaas who narrated that the Prophet (peace and blessings of Allaah be upon him) said:
Jibreel taught me one style and I reviewed it until he taught me more, and I kept asking him for more and he gave me more until finally there were seven styles.
(narrated by al-Bukhaari, 3047; Muslim, 819)
What is meant by styles (ahruf, sing. harf)?
The best of the scholarly opinions concerning what is meant is that there are seven ways of reciting the Qur’aan, where the wording may differ but the meaning is the same; if there is a different meaning then it is by way of variations on a theme, not opposing and contradiction.
Bilal Philips writes that the Qur’an continued to be read according to the seven ahruf until midway through Caliph ‘Uthman’s rule when some confusion arose in the outlying provinces concerning the Qur’an’s recitation. Some Arab tribes had begun to boast about the superiority of their ahruf and a rivalry began to develop. At the same time, some new Muslims also began mixing the various forms of recitation out of ignorance. Caliph ‘Uthman decided to make official copies of the Qur’an according to the writing conventions of the Quraysh and send them along with the Qur’anic reciters to the major centres of Islam. This decision was approved by Sahaabah and all unofficial copies of the Qur’an were destroyed. Uthman burned the unofficial copies of the Quran. Following the distribution of the official copies, all the other ahruf were dropped and the Qur’an began to be read in only one harf. Thus, the Qur’an which is available throughout the world today is written and recited only according to the harf of Quraysh. [Taken from Wikipedia]
The Seven Recitations:
With regard to the seven recitations (al-qiraa’aat al-saba’), this number is not based on the Qur’aan and Sunnah, rather it is the ijtihaad of Ibn Mujaahid (may Allaah have mercy on him). People thought that al-ahruf al-saba’ (the seven styles) were al-qiraa’aat al-saba’ (the seven recitations) because they happened to be the same number. But this number may have come about coincidentally, or it may have been done deliberately by Ibn Mujaahid to match what was narrated about the number of styles (ahruf) being seven. Some people thought that the styles (ahruf) were the recitations, but this is a mistake. No such comment is known among the scholars. The seven recitations are one of the seven styles, and this is the style that ‘Uthmaan chose for all the Muslims.
Read more here.
Still confused? Go through this article as it explains the whole thing in very easy language. :)