Hadith are traditions relating to the sayings and doings of the Islamic prophet Muhammad. Hadith collections are regarded as important tools for determining the Sunnah, or Muslim way of life, by all traditional schools of jurisprudence. The overwhelming majority of Muslims consider hadiths to be essential supplements to and clarifications of the Quran, Islam's holy book.

In the matter of what is called fiqh, or Islamic jurisprudence, the Quran contains many rules for the behavior expected of Muslims. However, there are many matters of concern, both religious and practical, on which there are no specific Quranic rules. Muslims believe that they can look at the way of life, or sunnah, of Muhammad and his companions to discover what to imitate and what to avoid.

In the matter of what is called tafsir, or exposition of the meaning of the Quran, Muslim scholars believe that it is useful to know how Muhammad or his companions explained the revelations, or upon what occasion Muhammad received them. Sometimes this will clarify a passage that otherwise seems obscure.

Hadith are a source for Islamic history and biography. For the vast majority of devout Muslims, authentic hadith are also a source of religious inspiration.

Muslims who accept hadith believe that trusted hadith are in most cases the words of Muhammad and not the word of God, like the Quran. Hadith Qudsi form a partial exception; this small minority of hadith purports to express words spoken by God to Muhammad but not included in the Quran, or the sense of them.

While both hadith and Quran have been translated, most Muslims believe that translations of the Quran into another language other than Arabic are inherently deficient, amounting to little more than a commentary upon the text. There is no such belief regarding hadith. Practicing Muslims cleanse themselves (wudu) before reading or reciting the Quran; there is no such requirement for reading or reciting hadith. Even for Muslims who accept the hadith, they are clearly of inferior rank.
 

How Hadith were collected and evaluated:
 

Traditions regarding the life of Muhammad and the early history of Islam were passed down orally for more than a hundred years after the death of Muhammad in 632.
 

Muslim historians say that it was the caliph Uthman (the third caliph, or successor of Muhammad, who had formerly been Muhammad's secretary), who first urged Muslims both to write down the Quran in a fixed form, and to write down the hadith. Uthman's labors were cut short by his assassination, at the hands of aggrieved soldiers, in 656.
 

The Muslim community (ummah) then fell into a prolonged civil war, termed the Fitna by Muslim historians. After the fourth caliph, Ali ibn Abi Talib, was assassinated, control of the Islamic empire was seized by the Umayyad dynasty in 661. Ummayad rule was interrupted by a second civil war (the Second Fitna), re-established, then ended in 758, when the Abbasid dynasty seized the caliphate, to hold it, at least in name, until 1258.
 

Muslim historians say that hadith collection and evaluation continued during the first Fitna and the Umayyad period. However, much of this activity was presumably oral transmission from early Muslims to later collectors, or from teachers to students. If any of these early scholars committed any of these collections to writing, they have not survived. The histories and hadith collections we possess today were written down at the start of the Abbasid period, more than one hundred years after the death of Muhammad.
 

The scholars of the Abbasid period were faced with a huge corpus of miscellaneous traditions, some of them flatly contradicting each other. Many of these traditions supported differing views on a variety of controversial matters. Scholars had to decide which hadith were to be trusted as authentic narrations and which had been invented for various political or theological purposes. For this purpose, they used a number of techniques which Muslims now call the science of hadith.
 

The most common technique consists of a careful examination of the isnad, or chain of transmission. Each hadith is accompanied by an isnad: A heard it from B who heard it from C who heard it from a companion of Muhammad. Isnads are carefully scrutinized to see if the chain is possible (for example, making sure that all transmitters and transmittees were known to be alive and living in the same area at the time of transmission) and if the transmitters are reliable.
 

Hadith that were not thrown out as clearly spurious (maudu') were usually sorted into three categories:
 

"genuine" (sahih, the best category)
"fair" (hasan, the middle category)
"weak" (da'if)

Some of the sahih hadith were further distinguished as mutawatir, or agreed upon. The sayings or events reported in these hadith were attested by so many witnesses, though different isnads, that it was thought inconceivable that these hadith could be forgeries.
 

More information on Hadith may be found at:
 

http://www.usc.edu/dept/MSA/fundamentals/hadithsunnah/

http://en.wikipedia.org/wiki/Hadith#Types_of_hadith