How is marriage different from a civil union?

A civil marriage is officiated by a justice of the peace, a judge, or some other civil authority. A religious marriage is officiated by a clergy member.

United States

Marriage is a civil legal status in the US and before any marriage, civil or religious, can be solemnized the happy couple must obtain a civil marriage license in the state where they plan to marry. Without a civil marriage license signed by the officiating clergy, a religious marriage would not be a legal marriage. The state allows various officials both religious and civil to perform the ceremony but it is only valid when performed pursuant to a civil marriage license. It's the law.

A religious marriage is governed by all the state and Federal Laws that govern marriage in the US and by all the rules and doctrines of the particular denomination to which the couple belongs. A common aspect is male dominion in the marriage. The Catholic Church, one of the largest religious organizations in the world, does not allow divorce within the church. It considers marriage to be a life long commitment.

A civil marriage is subject to all the state and federal laws that govern marriage and no more. Each party has equal rights. If either party wants a divorce they only need to file a complaint for divorce at the local family court and they are free to end their marriage legally. The state will ensure an equitable division of any marital property and determine custody and child support arrangements for any children born of the marriage.