A diocese is a jurisdiction of a bishop or pastor. People within a certain diocese fall under the pastoral care of the bishop of that district.
Originally, the Roman Empire was divided into dioceses, which were areas of land, so divided for administrative purposes. When Christianity became the official Roman religion, each diocese was appointed a bishop to oversee the ministry in that area. As the Roman ecclesiastical system spread to other regions, the division of cities into religious dioceses continued.
A diocese is divided into smaller parishes, with each parish assigned a priest who ministers there. All the parish priests in one diocese are supervised by a bishop. Several dioceses are grouped together to form an archdiocese, which is supervised by an archbishop. In the Roman Catholic Church, the United States is divided into over 17,000 parishes with 144 dioceses and 32 archdioceses, as well as one archdiocese for the Military Services, USA. There are an additional 16 dioceses and 2 archdioceses in the Eastern Catholic Church (www.usccb.org/about/bishops-and-dioceses#tab--episcopal-regions-archdioceses-and-dioceses-in-the-us, accessed 2/8/22). Because of the vast number of congregants and churches, many major metropolitan areas have an archbishop as well, even though the city may not be technically considered an archdiocese. Above the archbishops are the cardinals and finally the pope.
Today, most people think of dioceses within the Catholic Church, but several other churches, including the Episcopal Church (100 dioceses in the U.S. grouped into 9 provinces), have a similar hierarchical arrangement with dioceses and archdioceses. Other churches, such as the Evangelical Lutheran Church in America, have a hierarchical arrangement with bishops and archbishops, although their responsibilities may not fall along the geographical boundaries of a diocese or archdiocese.