He's the Bishop of Rome, and Leader of the Western Branch of Catholic Christianity.

It used to be that each Bishop was the autonomous ruler of his church, and there used to be four big ones who had huge churches with lots of power: 1) Bishop of Rome, 2) Bishop of Constantinople, 3) Bishop of Antioc, and 4) Bishop of Alexandria, but there were lots of other Bishops with varying degrees of power.

Some time around the 11 century the eastern and western churches got into a dispute over which Bishop, if any, had, or should have, the most authority, and it divided roughly along lines corresponding to the political domains in which their churches resided.

The western group thought there should be one Bishop with more authority than the others, and the eastern group thought it should continue the way it always had been, so they split, and because, of the big Bishops, only one of them, the Bishop of Rome, was in the western group, he became the defacto leader of the western group.

For awhile there was a dispute in the western group, where there were two Popes each claiming to be the great leader of the western group, but eventually they sorted it out and got back to having just one.

The leader of the Catholic church...

The Pope (from Latin: papa, father; from Greek (papas) = father - originally written (pappas), as in Homer's Odyssey, book VI, line 57) is the Bishop of Rome, the spiritual leader of the Roman Catholic Church and head of state of Vatican City. Faith communities which recognize Apostolic Succession acknowledge the Bishop of Rome as successor of St Peter. As such, Catholics believe the pope to be the Vicar of Christ, while the other faith communities disacknowledge Petrine primacy among the bishops. The office of the pope is called the "papacy"; his ecclesiastical jurisdiction is called the "Holy See" (Sancta Sedes in Latin) or "Apostolic See" (this latter, on the basis that both St. Peter and St. Paul were martyred at Rome). Early bishops occupying the See of Rome were designated "Vicar of Peter"; for later popes the more authoritative Vicar of Christ was substituted; this designation was first used by the Roman Synod of 495 to refer to Pope Gelasius I, an advocate of papal supremacy among the patriarchs. Marcellinus (d. 304) is the first Bishop of Rome whom sources show used the title of pope. In the 11th century, after the East-West Schism, Gregory VII declared the term "Pope" to be reserved for the Bishop of Rome. The current (265th) pope is Pope Benedict XVI, who was elected April 19, 2005 in papal conclave.