What is the difference between a basilica... a cathedral... an oratory and a church?
In the Roman Catholic Church, a basilica is a designation for an important church building. A basilica is designated by the pope to buildings that carry special spiritual, historical, and architectural significance. Once a basilica -- always a basilica. A basilica may or may not also be the cathedral of the diocese. This is the highest permanent designation for a church building. The word basilica derives from a style of church based on the old roman basilicas which were houses of Law.
A cathedral is a church which holds a bishop's throne (called cathedra). It means it is the central church of his diocese. A cathedral may or may not be a basilica. It is the home church for the bishop or archbishop of a Catholic diocese. A cathedral may not always be a cathedral, depending on the decisions of the bishop. Hierarchally, the cathedral is the most important church of a diocese. A church is a building in which the public sacrifice of the Mass takes place. An oratory is more like a chapel, which is a place where people celebrate private Masses. An oratory is a structure other than a parish church, set aside by Church authority for prayer and the celebration of Mass. Examples would be chapels in airports and colleges where all of the functions of a parish aren't required. Private chapels are permitted in Catholicism, but if Church authority had not sanctioned their particular creation, they are chapels, not oratories. All of these are churches in that they have a consecrated altar and are used for celebrating the Sacraments.