Hadith literally means saying. And refers to sayings of Prophet narrated by people. After Prophet SAWW there was bans on narration and writings of hadith and first hadith collections were by Bukhari and Kulayni when this ban was removed. 

Most of hadith have usually 4-6 persons in their chain of narrators to reach prophet SAWW. 


According to Shia Islam sayings of any of the fourteen infallibles are considered same as sayings of prophet SAWW because Shia believe prophet SAWW left his divine knowledge for Imam Ali a.s. and he left for second Imam (Imam Hasan a.s) and then left for Imam Hussain a.s. and so on and currently this knowledge is with [Imam Mahdi][1] a.s. who is in Occultation. 


  [1]: http://en.wikipedia.org/wiki/Muhammad_al-Mahdi