"Husbands, love your wives, just as Christ loved the church and gave himself up for her. -Ephesians 5:15
At its most basic level, marriage is a union between a man and a woman for the purpose of procreation and mutual love and support.
In the Catholic Church, marriage is considered to be more than a natural institution. It was elevated by Christ Himself, in His participation in the wedding at Cana (John 2:1-11), to be one of the seven Sacraments. A marriage between two Christians, therefore, has a supernatural element as well as a natural one.
The ministers of the Sacrament of Marriage are the spouses themselves, because the mark - the external sign - of the Sacrament is not the wedding Mass, but the marriage contract itself. This does not mean the wedding license that the couple receives, but the vows that each spouse makes to the other. As long as each spouse intends to contract a true marriage, the Sacrament is performed.
The effect of the Sacrament is an increase in sanctifying grace for the spouses, a participation in the divine life of God Himself.