Short answers, pulled from the story.
Sunni Islam is the largest branch of Islam and the largest religious denomination in the world. A Pew Research Center study conducted in 2010 and released in January 2011 counted 1.62 billion Muslims, of whom an estimated 85 to 90 percent are Sunni.
Sunnis hold that Muhammad appointed no successor and that his closest companion Abu Bakr rightfully became caliph, appointed at the meeting of Saqifa. This contrasts with the Shia view that Muhammad appointed Ali ibn Abi Talib as his successor.
The term Sunni means those who observe the sunnah, the practices of Muhammad. The Arabic word sunna dates back to pre-Islamic language, where it meant the right path that has always been followed.
The four Sunni legal schools are Hanafi, Hanbali, Maliki, and Shafi'i. Their founders, Abu Hanifa, Malik ibn Anas, Shafi'i, and Ahmad bin Hanbal, all practiced during the Abbasid period, and the four schools recognize each other's validity.
Sunni theology comprises three traditions: the Ash'ari school, named after Abu al-Hasan al-Ash'ari, who died in 935; the Maturidi school, named after Abu Mansur al-Maturidi; and the textualist Athari school, which rejects rationalistic kalam in favor of strict textualism.
The caliphate was constitutionally abolished in Turkey on the 3rd of March 1924, following the fall of the Ottoman Empire at the end of World War I. The Ottoman Empire had been the biggest Sunni empire for six centuries.
The six pillars of iman are belief in the Oneness of God, belief in the Angels of God, belief in the Holy Books, belief in the Prophets of God, belief in Resurrection after death and the Day of Judgment, and belief in Preordainment, known as Qadar.