"A diocese is a portion of the People of God which is entrusted to a bishop for him to shepherd in cooperation with the presbyterium."
- Code of Canon Law, 369

Put simply, a diocese is a geographical area which breaks up the Roman Catholic Church into particular areas, which are each governed and overseen by a bishop alongside the priests. A diocese is named based on where the Cathedral church (the bishop's church) is located.

The Diocese of Joliet encompasses a seven-county area in northern Illinois: DuPage County, Will County, Grundy County, Kendall County, Kankakee County, Ford County, and Iroquois County.

A diocesan priest is just what he sounds like - he is a priest that is active in his ministry almost always within that geographic area (in contrast to religious priests, whose areas for ministry are sometimes the whole world wide).

In the Diocese of Joliet, the majority of our priests are assigned to serve as parish priests (either pastors or associate pastors) somewhere within those seven counties.

Some diocesan priests do other ministry as well, either in addition to or instead of a parish assignment. Joliet priests are also teachers, school administrators, seminary professors, police and fire chaplains, nursing home and hospital chaplains, and directors of ministries.