It's a fancy word for marriage; that's why they say it at weddings.
wedding rings
It depends. A sacrament is an earthly symbol with a heavenly meaning. As an example - the water of baptism - water is an earthly symbol but has a heavenly meaning - that of repentance through spiritual cleansing, and rebirth into a new Christian life. If you are a Roman Catholic, there are seven sacraments: Baptism, Confirmation, Holy Communion (mass), Matrimony, Ordination, Penance and Unction (prayer for the sick). In other denominations only Baptism and Holy Communion are classed as sacraments as these were instigated (in the case of Holy Communion) and endorsed (in the case of Baptism) by Christ himself.