Hadith is the collection of sayings of Ahlul Bayt
Shia” is the short form of the historic phrase Shīʻatu ʻAlī (شيعة علي), meaning “followers of Ali”, “faction of Ali”, or “party of Ali”.
Shias do not consider Ali to be the fourth Caliph, rather the first “Imam”. Shias believe that there are numerous narrations where the prophet selected Ali as his successor.
Muhammad’s (Sallallahu Alaihi Wasallam) family, the Ahl al-Bayt (“the People of the House”), and certain individuals among his descendants, who are known as infallible Imāms, have special spiritual and political authority over the community.
Thus the Qurʻān and Hadith interpretation and differences in Hadith narrators are the main distinction of the Shias.
The Grandsons of the Prophet Muhammad, Hasan ibn Ali and Husayn Ibn Ali are agreed upon by all Muslims to be the “leaders of all youths in Paradise.” Shias also believe that these sons of Imam Ali were the true leaders and caliphs of the Muslims.
Each Imam was the son of the previous Imam, with the exception of Husayn ibn Ali, who was the brother of Hasan ibn Ali.
The twelfth and final Imam is Muhammad al-Mahdi, who is believed by the Twelvers to be currently alive, and hidden until he returns to bring justice to the world.