LDS Church explains Mormon lay ministry

Virtually every member of The Church of Jesus Christ of Latter-day Saints (Mormons) is given an opportunity to offer service in their local congregation, a geographically designated group called a ward or branch (based on local population). Several wards and branches combine to form a stake (a broader geographical boundary) and an additional layer of administrative responsibility. In a church with lay leadership, the work of the individual congregations depends wholly on the volunteer efforts of the local members.
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