These have connections, but are not identical, to the differing versions of Islam, which The Economist summarized in an article in the same issue on The New Strife.
"The two branches of Islam split during the great fitna, or strife, over the succession to the Prophet Muhammad. Sunnis claim that the leadership passed down the line of the four rashidun (rightly guided or perfect) caliphs who had been the Prophet’s companions: Abu Bakr, Omar, Uthman and only then to Ali, the Prophet’s cousin and son-in-law. After that the mantle passed to the Umayyads in Damascus, followed by the Abbasids in Baghdad. Shias say the succession was usurped. It should have passed through the family of the Prophet, first to Ali and later Hussein. But Ali was murdered in Kufa and buried nearby in Najaf, whereas Hussein was killed in a battle against the Umayyads in Karbala and buried there—hence the importance to Shias of the two cities. Shia leadership then passed down a chain of imams that broke off at different points, according to their sect—eg, the Zaydi 'Fivers', the Ismaili 'Seveners' and the majority 'Twelvers'."