Hi, Steve —

An archdiocese is a diocese that serves as the head of a metropolitan province or metropolia which encompasses several other dioceses, called suffragan dioceses. These dioceses together form a type of region, much like a large metropolitan area, with the archdiocese at the head.

For example, the archbishop of Boston is metropolitan over all the New England dioceses and bishops. In Eastern churches, the archdiocese often exercises real authority over the bishops in his metropolia. In the Roman Catholic dioceses that are common in the U.S., however, this leadership role is merely honorary and carries few or no real privileges.

An archbishop is the head of an archdiocese, and a bishop is the head of a diocese (or an auxiliary to a diocese or archdiocese). He is sometimes called (more often in the Eastern churches)
a metropolitan, emphasizing his leadership over the metropolitan province.

Bishops can also acquire the title archbishop by virtue of their office (such as a papal nuncio, or ambassador from the Holy See) or as a personal, honorary title.

Eric