A titular church is a church assigned to a cardinal, who becomes the titular priest or deacon of that church. These titles do not refer directly to the familiar categories of priest and deacon in the sacrament of Holy Orders of the Roman Catholic Church, and in fact pre-date the developed understanding of the sacrament.
In the early centuries the cardinals (the word is from the Latin cardo, "hinge") made up the administrative body of the primitive Roman church under the authority of the Bishop of Rome. They fell into three categories: Priests responsible for individual places of worship (the original tituli), deacons responsible for centres of charitable activity (the so-called diaconiae) and bishops of the suburbicarian dioceses which were recognised as being under the authority of the Bishop of Rome. This tradition had been symbolically preserved by appointing present-day cardinals as titular priests and deacons of Roman churches, even though most of them are bishops elsewhere.
The titles used may differ in wording from the names of the churches concerned, and in a few examples are completely different. For instance, the title of Santa Francesca Romana is still the church's old name, Sancta Maria Nova. It is best not to assume that a title is automatically the official name of the church concerned.
Once a church is made titular, it retains that dignity unless a pope formally revokes its status, even if there is currently not a cardinal attached to it. Some titular churches have gone decades without an incumbent cardinal.
When the status of titular church is given, it is specified whether it is a presbyterial or a diaconal title. At times, a cardinal is promoted to the rank of Cardinal Priest but still retains his titular deaconry, which is promoted with him to a presbyterial title pro hac vice ("for the occasion"). This arrangement lapses with the transfer or death of the incumbent.