WHAT IS A BASILICA?
A basilica is a building which, because of its artistic, architectural, spiritual, or historic significance, is given a special honorific designation and privileges by the Pope. The four “Major Basilicas” are in Rome, while there are what are titled “Minor Basilicas” all over the world. The city of Barcelona presently possesses nine such churches which have been so honored with this title by the Popes over the centuries.
To read more about how a church becomes a basilica, visit the entry covering this topic in the Catholic Encyclopedia