Islam is the second-largest religion in the world, with 1.6 million followers as of 2012. It was founded in Mecca and Medina by the Prophet Muhammad, in the 7th century AD. The founding lore of Islam, as detailed in their holy book, the Quran, states that God picked Muhammad, a nomadic merchant, to be a prophet. Muhammad was approached by angels and received a series of revelations from God over 23 years which would shape the faith and be noted down in the Quran. Muhammad then began to preach this message and convert people.
Muhammad preached in the city of Mecca but was driven out, and escaped to the nearby city of Medina, where he became a judge. Muhammad, who was recognized as a prophet, then became a military leader and went on to conquer Mecca before he died in 632 AD.
After Muhammad died, there was an argument over who should succeed him. Some favored his close friend Abu Bakr, others favored his cousin Ali. This split resulted in a fracture among Muslims, between the Sunnis, who favored Abu Bakr, and the Shiites, who favored Ali. Abu Bakr won out and along with three others who followed him, was called a 'Rashidun' or 'rightly guided' Caliph. The Caliphs, Muslim religious and political leaders, conquered all of Arabia, Persia, the Levant (present-day Israel), Egypt, and parts of North Africa in 30 years, forming a religiously-run empire, or Caliphate.
The Caliphate would reach its largest extent under the Umayyad dynasty in the 8th century, at which point it stretched from Spain to India, and was one of the largest empires in history. This kicked off the Golden Age of Islam which lasted for 400 years. In this time, great scientific and medical advances were made in the Muslim world. Although the Golden Age was interrupted by the crusades, a series of holy wars launched by Christian nations to attempt to retake the Levant between 1000 and 1200 AD.
The Mongols, nomadic horsemen from Asia, invaded in the 13th century and ended the Golden Age. At this point, the Muslim world was fractured, until the 14th and 15th centuries, when the most famous Muslim power, the Ottoman Empire, arose. In this period, Muslims converted Indonesia and parts of India to Islam as well, often through brutal conquest.