The caliph is the highest title in islam, however it was abolished in 1924 by Mustafa Kemal Ataturk, who was the president of turkey. He abolished it because the royal family of turkey had been hereditary caliphs of islam since 1517, and he was anti-monarchist. Once the title of caliph had been abolished there was no other hierarchy for islam since everybody else was appointed by the caliph, there were only local religous leaders left. There is no current head of islam, even though there have been several conferences about appointing ine since, but not all countries with a predominantly muslim population accept the idea and so no agreement has been reached.
There is currently no head of islam