A "hadith" is a "saying". When used in the context of Islam, a hadith means "a report of what the Holy Prophet Muhammad said or did on a certain occasion".

Hadith refer to the record of the words, actions, and the silent approval of the prophet Muhammad (May Allah's peace and Blessings be upon Him) (see also in Wikipedia). Hadith may also be known as Sunnah, though sunnah has some differences, Hadith is part of the sunnah. There are two categories of Hadith:

  1. Hadith Qudsi
  2. Hadith Nabawi

Hadith Qudsi are the words of Allah in the words of the Prophet. Hadith Nabawi are the sayings of the Prophet himself. Ahadith have been collected by many, there are six major collections of Ahadith and they are:

  • Sahih Bukhari collected by Imam al-Bukhari
  • Sahih Muslim collected by Muslim b. al-Hajjaj and who was a Student of al-Bukahri.
  • Sunan Abu Dawood, collected by Abu Dawood al-Sijistani
  • Jami'ul Tirmidhi collected by Abu'Isa At-Tirmidhi a Student of imam al-Bukhari.
  • Sunan An-Nasa'i collected by Ahmad ibn Sho'aib An-Nasa'i a Student of imam al-Bukhari and his students.

As for the sixth there's a difference of opinion on which to chose:

  • Al Muwatta' collected by Imam Malik ibn Anas who died 179 a.H..
  • Sunan ibn Majah collected by Abu 'Abdullah Ibn Majah a Student of abu Bakr ibn Abi Shaybah a teacher of al-Bukahri, Muslim, imam Ahmad and abu Dawod.

Some useful questions on Islam.SE regarding Hadith: