The Latin word basilica (derived from Greek, Basiliké Stoà, Royal Stoa), was originally used to describe a Roman public building, usually located in the forum of a Roman town. After the Roman Empire became officially Christian, the term came by extension to refer to a large and important church that has been given special ceremonial rites by the Pope.
The Early Christian purpose-built basilica was the cathedral basilica of the bishop. Basilicas in this sense are divided into classes, the major ("greater"), and the minor basilicas, i.e., three other patriarchal and several pontifical minor basilicas in Italy, and over 1,400 lesser basilicas on all continents.
Major or papal basilicas belong just four great papal churches of Rome. Upon relinquishing the title of Patriarch of the West, Pope Benedict XVI renamed these basilicas from "Patriarchal Basilicas" to "Papal Basilicas". The lesser minor basilicas are the vast majority, including some cathedrals, many technically parish churches, some shrines, some abbatial or conventual churches. Some oratories, semi-private places of worship, have been raised to the status of minor basilica.
Basilica Wikipedia Entry