The word “Sunni” in Arabic means “one who follows the traditions of the Prophet.” The word “Shia” in Arabic means “a group or supportive party of people.” They are also known as “followers of the Prophet.”
Shias believe that Imams are descendants of the Prophet. Sunnis accept that the first four Caliphs were followers of Mohammad, but do not grant any kind of divinely inspired status to their clerics. Afghanistan is made up of 84 percent Sunnis and 15 percent Shias. Most Pashtuns are Sunni Muslims, while most Hazaras are Shia Muslims.
In “The Kite Runner” Amir and Baba are Sunni Muslims and Hassan and Ali are Shia Muslims. Hazaras are generally Shia Muslims and Pashtuns are generally Sunni Muslims. They have different beliefs but between Amir and Hassan it does not make a difference. Hassan is called derogatory names by Assef and some others because he is a Hazara and a Shia Muslim. Amir is not because he is a Pashtun and a Sunni Muslim.