The Shi'a belief on 'Imamat'
The Shiites believe that after the death of Prophet Muhammad (pbuh) and this till the end of the world, Allah has nominated twelve Imams. According to them, those twelve Imams had been nominated by Allah through Prophet Muhammad (pbuh). The first Imam according to them is Ali (ra), the second one, his son, Imam Hassan, the third one Imam Hussein and the rest of the nine Imams being found in their bloodline.
From the first Imam (i.e. Ali) till the eleventh Imam (Hassan Askari), all died a natural death like the rest of mankind. As for the twelfth Imam, the son of Hassan Askari, he mysteriously disappeared in this youth and went into hiding in a cave. Now till the end of the world, according to the Shiites, he alone has the exclusive right to reign upon the world. The belief in Imamat is obligatory (Farz) for a Shiite and the one who does not believe in it shall see himself being rewarded hell as resting place. In one of their books, Al-Kafila there is the mention that if the earth (i.e. the world) remain without an Imam, it shall sink. Still according to this book, the (Shiites) Imams are selected by Allah just like Allah chose the prophets.
Shia in itself is not a religion; it is a sect in Islam. After the martyrdom of Hazrat Umar (ra) and during the caliphate of Hazrat Uthman (ra), Islam propagated in several countries outside Arabia, such as Persia, etc. Many people accepted Islam at that time. Many entered the fold of Islam sincerely while many others entered therein hypocritically.