The Holy See dates back to Early Christianity and is the principal episcopal see of the Catholic Church, which has approximately 1.329 billion baptized Catholics in the world in the Latin Church and 23 Eastern Catholic Churches. The independent state of Vatican City, on the other hand, came into existence on 11 February 1929 by the Lateran Treaty between the Holy See and Italy, which spoke of it as a new creation, not as a vestige of the much larger Papal States (756–1870), which had previously encompassed much of central Italy. (Full article...)
The Papal State(s), the State(s) of the Church, the Pontifical States, the Ecclesiastical States, or the Roman States (Italian: Stato Pontificio, also Stato della Chiesa, Stati della Chiesa, Stati Pontifici, and Stato Ecclesiastico; Latin: Status Pontificius, also Dicio Pontificia) were among the major historical states of Italy from roughly the 6th century until the Italian peninsula was unified in 1861 by the Kingdom of Piedmont-Sardinia (after which the Papal States, in less territorially extensive form, continued to exist until 1870).
The Papal States comprised territories under direct sovereign rule of the papacy, and at its height it covered most of the modern Italian regions of Romagna, Marche, Umbria and Lazio. This governing power is commonly called the temporal power of the Pope, as opposed to his ecclesiastical primacy.
The plural Papal States is usually preferred; the singular Papal State (equally correct since it was not a mere personal union) tends to be used (normally with lower-case letters) for the modern State of Vatican City, an enclave within Italy's national capital, Rome. The Vatican City was founded in 1929, again allowing the Holy See the political benefits of territorial sovereignty.
The Papal Basilica of Saint Peter (Latin: Basilica Sancti Petri), officially known in Italian as Basilica Papale di San Pietro in Vaticano and commonly known as Saint Peter's Basilica, is a Late Renaissance church located within the Vatican City.