Question: What is a Caliph?

Answer: Caliph means successor, as in the successor of the Prophet (s) 

Question: What is the difference between the caliphate in Sunni Islam and Shia Islam?

Answer: The caliphate in the former is largely a political leadership. A caliph is Sunni Islam is selected by humans. In Shia Islam, the primary function of a caliph is religious and he can be selected by the Prophet Muhammad (s) and God only.

Question: What is nass?

Answer: Nass literally means designation, as in the designation of an Imam and Caliph by God. 

Question: According to Sunni Islam, can someone appoint himself as a Caliph?

Answer: Yes, as long as there are people around who are willing to give him their oath of allegiance. As long as he declares himself a Muslim, he can do this. 

Question: Who were the first Caliphs in Sunni and Shia Islam?

Answer: In Sunni Islam, the first Caliph was Abu Bakr, whereas in Shia Islam it was Imam Ali (as).