The original basilica was built by Constantine back in the 4th century AD.  Over the centuries, fires and earthquakes and invaders have ravished the building and it's undergone many renovations and repairs.  It's always maintained its status as the Mother Church to Christianity though. 

The bronze main doors of the basilica which were originally on the Senate chamber of the Curia in the Roman Forum.

The Jubilee Door or Holy Door.

The nave of the basilica.  Of all the basilicas and churches in Rome, this one most accurately keeps the classical layout of a Roman basilica.  Even though there's been many renovations and modifications over the centuries, the form has been kept essentially pure.

The ceiling of the nave.

The 14th century Cosmateque floor is probably the best example of that art in Rome.

The baldaccino or canopy over the main altar is from 1369.  The two gold busts in the cage at the top are of Saints Peter and Paul and are reputed to contain portions of the skulls from the saints.  The altar table contains some pieces of wood from the table at St. Pudens' house that was used by Peter as an altar there.

The mosaics in the apse date back to the 13th century and were created by two Franciscan monks.

This is the Cathedra or Episcopal Throne which makes the basilica the Cathedral of Rome.

The choir and organ date back to the 19th century.

My favorite is the Baroque-style Lancellotti Chapel created at the orders of Pope Innocent X in 1650.

The ceiling vault of the Lancellotti Chapel.

The Altar of the Holy Sacrament.  The altar table is said to contain fragments of the cedar table used at the Last Supper by Jesus and the Apostles in Jerusalem.  The marble and bronze columns were originally on the Temple of Jupiter on the Capitoline Hill.  They were fabricated from the melted-down prows of Cleopatra's fleet.

The tomb of Pope Leo XIII from 1907.  There are a total of six popes entombed in the basilica.

A stock picture that everyone makes.  Hundreds of shots of this floating around the internet.  The Cosmateque flooring in the left aisle giving a very illusory effect.  This floor is absolutely flat but it does give one a funny sensation to walk over it.