An ecclesiastical province is basically one large archdiocese and all the dioceses that are historically associated with it. Generally, in the Latin Rite in the United States, there is one ecclesiastical province per state, with a few exceptions for historical reasons. Certain liturgical matters, such as the decision whether to transfer the celebration of the Feast of the Ascension of Our Lord to Sunday, are left up to each ecclesiastical province.
In the Eastern Catholic Churches, ecclesiastical provinces are known as metropolitanates, and because of the smaller number of Eastern Catholics, generally cover a larger geographical area than just one state.
In practical terms, ecclesiastical provinces today fall between the level of the diocese and the national bishops' conference. They ensure a uniformity of liturgical celebration among local churches (dioceses) that are connected by geography and history.