St. Peters basilica was built over several centuries with many different architects and under many Papacies.
The basilica we see today was begun in 1506 with design changes and construction
under the direction of 15 different architects working under 22 different Popes over a period of about 155 years.

The original design was a Greek Cross plan (equal arms) by Bramante, who began construction in 1506 under the
warrior pope, Julius II della Rovere, who began tearing down the original Constantinian basilica built for Constantine between 313 and 339.

St. Peters is not the Cathedral of Rome, as that honor and title belong to the church of St. John Lateran.
St. Peters is the presumed burial site of the Apostle Peter, and built over his alleged grave,
and because of that, it has always been a popular pilgrimage site throughout Christian history.
It is know as a "basilica",which is a form of architecture used by the Romans,
and adopted by the early Christians for their churches,

It is interesting to note that while Bramante was working on the first construction of the St. Peters
beginning in 1506, that Michelangelo was frescoing the Sistine Chapel, (1508-1512),
and Raphael and his assistants were working on frescoes in the Pope's private apartments known as the "Stanze" (1509-13).
All three of these "greats" in the history of art and of the Catholic Church, were working within a few meters of each other
at the same time, in the Vatican and St. Peters.