After the death of Muhammad, the Muslim people began looking to a new type of ruler, which they called a caliph. The word ‘caliph’ means successor. The caliph was the governmental successor to Muhammad. He was not, however, the prophetic successor, since Muslims believed that Muhammad was the last prophet.
Under the direction of the caliphs, the Islamic empire continued to spread in influence, wealth and power. The first four caliphs were close friends and relatives of Muhammad. Through their military might they greatly expanded the borders of Islam, bringing their once great neighbors, the Persians, completely under their control. These leaders also conquered much of the Byzantine empire.