Chains: According to Roman tradition, St. Peter was kept in chains in the dungeon (tullianum) of the Mamertine prison. He was miraculously released and was safely on his way out of town, the legend continues, when he met Christ who was heading into Rome to share the suffering of Christians being martyred in Nero's persecution. Peter turned back to join the ranks of the martyrs. He was taken to the Roman Urban Prefecture (police headquarters) at the base of the Esquiline Hill and again fettered before eventually being killed in Nero's stadium in the Vatican district.
The two sets of chains that Peter wore took a circuitous route to the Church of St. Peter in Chains (S. Pietro in Vincoli) near the site of the Prefecture, going first to Constantinople when the Roman capital was transferred there. There are also accounts of chains being presented to the Empress Eudoxia, the wife of Theodosius II by Bishop Juvenal of Jerusalem in 439 AD. She is said to have enshrined one set in the Basilica of the Apostles in Constantinople and sent the other to her daughter (another Eudoxia) in Rome where, several years later, they were displayed in a new church that eventually acquired the name St. Peter in Chains. (How Juvenal came to have the chains in Jerusalem is not clear.) At any rate, the Constantinople chain eventually was also sent to Rome where, according to legend, the two chains miraculously united into the one that is displayed in an exquisite reliquary in the main altar of the church.
Such pious legends are difficult for educated 21st century people to believe, but Christians venerated the chains for most of the last 1500 years. Pilgrims in the middle ages brought filings from the chains back to their home countries, and churches of St. Peter in Chains proliferated all over Europe to house the relics. Eventually, some such relics were moved to other parts of the world, including the Americas.
The first church built on the site was started in 442 by the younger Eudoxia and consecrated by Pope Leo I. It was completely rebuilt in the eighth century and extensively remodeled and redecorated in the 1470's. It still has its original shape and the twenty fluted columns with Doric capitals are ancient if not original -- some authorities say they came from the Prefecture. Art and architecture critics are almost unanimous in the opinion that the church was ruined by Meo del Caprina, who directed the 15th century project for Pope Sixtus IV. Only Caprina's front portico escapes the general opprobrium.
Moses: The pious may go to this church to venerate the chains, but the big tourist attraction is Michelangelo' Moses, the centerpiece of his monumental tomb for Pope Julius II, which originally was to have been placed in St. Peter's in the Vatican. Michelangelo never finished the tomb, because Julius and his successors constantly encumbered the artist with other, higher priority work, not least of which was the painting of the ceiling of the Sistine Chapel in the Vatican.
The much reduced and incomplete tomb was placed (along with Julius) in the Church of St. Peter in Chains by his successors, who refused him the dignity of internment in the Vatican Church of St. Peter -- he was much noted for his "terribilita" and was never well liked. For years, it was thought that only the Moses and perhaps the statues of Aaron and Joshua on the upper level were the work of Michelangelo and that other parts were done by his students. But during restorations in 1999, experts found evidence (skill and technique) that the much smaller figure of Julius on the top of the sarcophagus may also be Michelangelo's work. The six slaves, possibly by Michelangelo, that were to have adorned a platform for the tomb are displayed at the Galleria dell'Accademia in Florence (4) and the Louvre in Paris (2). There are other notable works of art in the basilica, but tourists rushing in to see the Moses generally ignore them.
The Church is on Piazza S. Pietro in Vincoli near the Oppian summit of the Esquiline Hill. The easiest way to reach it is up a flight of covered stairs on the south side of Via Cavour, just west of where it intersects with Via degli Annibaldi. There may be crowds around the Moses, so an early morning arrival is best. The lights on the statue are metered and you have to feed in 500 Lira coins.
The Blue Guide "City Guide to Rome" and Georgina Masson's "Companion Guide to Rome", both available at better bookstores in Rome and through Internet booksellers, give a good introduction to the Church. Monographic guides are also available, and some can be had from the stalls that set up in the Piazza. There are also some good Internet links:
A multi-page site that concentrates on the history of the chains: http://www.cptryon.org/hoagland/travels/stpeterchains/index.html
The very good Euroweb Gallery presentation on the tomb (where there is a link to a good bio note on Michelangelo, and you can click on the blue dots for more information about the tomb): http://gallery.euroweb.hu/html/m/michelan/1sculptu/giulio_2/giulio2.html
Michelangelo's Moses -- buy your own copy (smaller, less elegant copies are available at stalls all over Rome: http://www.eleganza.com/detailed/moses.html
Rome's other Moses, by sculptor
Prospero Bresciano in the Fountain (Mostra) of the Acqua Felice in Piazza
S. Bernardo -- inspired by the Michelangelo Moses, but so bad that Bresciano
supposedly died of shame: http://www.photo.net/photo/pcd0803/rome-moses-fountain-11.4.jpg