A diocese is an area of land under the charge of a bishop. He looks after the religious needs of the Christians who live there. The bishop organises the work of the Church in the area, and is responsible for all the ministers (priests) in his area. Another word for diocese is see, or even 'episcopal see'.
The diocese is the key geographical unit of religious authority in the church. Archbishops have an archdiocese. Inside each diocese are a number of parishes. Each parish is the area supervised by a local priest, and is part of the diocese.
As the Western Roman Empire fell during the 5th and 6th centuries the bishops of the Church took over many of the administrative roles of the former Roman prefects. The areas of ecclesiastical administration always coincided with those of the Roman civil administration.
A thousand years later, the Ottoman Empire conquered the Eastern Roman, or Byzantine Empire. The eastern bishops then continued as the Roman civil structure was stripped away. Many an ancient diocese, though later divided among several dioceses, preserved the boundaries of a long-vanished Roman administrative division.