By far the largest group within Islam - approximately 85% of the world's Muslims are Sunni. They are so predominant that it is perhaps easy to treat it as "orthodox" Islam.
On the death of Muhammad, a group of his companions elected Abu Bakr as his caliph, or representative. This institution of religious and political leadership of the Muslim community (ummah) continued, through the Rashidun (the Four Rightly Guided Caliphs), the Umayyads, the Abbasids, and the Ottomans, until the Caliphate was abolished in 1924 by the Turkish Grand National Assembly.
Apart from Iran, Iraq, Azerbaijan, and Bahrain, Sunnis are a majority in most Muslim communities - in the Middle East, Africa, the Indian sub-continent, South East Asia, and China.