There are many churches in Rome - and throughout the world - dedicated to the Virgin Mary. The largest one is the Basilica Papale (or Papal Basilica) of Santa Maria Maggiore near the Termini Train Station in central Rome.
As you might guess from the name, Santa Maria Maggiore is technically part of the Vatican - just as a foreign embassy might be. As part of Vatican City, the Basilica is also part of the UNESCO World Heritage Site that includes all extraterritorial properties of the Holy See in Rome.
Although the Basilica Papale di Santa Maria Maggiore has been expanded upon and redecorated over the centuries, it was originally built in the mid-5th century and much of the original structure is still in place. In the years after the papacy was moved back to Rome from Avignon, part of the church was used as the papal residence until renovations to the Vatican Palace was completed.
Some of the highlights of a visit to the basilica are its 5th century mosaics, a reliquary said to contain a piece of the crib that once held the baby Jesus, an ancient icon of the Virgin Mary, the tomb of Saint Jerome, and the tomb of the artist Gian Lorenzo Bernini. There is a Sistine Chapel in the Basilica Papale di Santa Maria Maggiore, named for Pope Sixtus V (the one in Vatican City is named for Pope Sixtus IV).