Saint Peter´s Basilica is located within the Vatican City in Rome. In Catholic tradition, it is the burial site of Saint Peter, who was one of the twelve apostles of Jesus and later first Bishop of Rome. The Basilica is famous as a place of pilgrimage as well as incredible masterpieces of art.

Sources, related links:
Wikipedia - St. Peter´s Basilica
Encyclopedia Britannica - Saint Peter´s Basilica