Civil marriage is a legal status defining the legal relationship between two persons (almost universally two persons of opposite sex). All governments (National, State, County, and City) make some statements about the rights, obligations, duties and privileges for persons who are married. Most commonly, these include power of attorney, inheritance rights, sharing finances, and rights in relation to any children they may have. With the advent of no-fault divorce, most of these rights and obligations can be terminated by either party at any time for any reason.
Civil marriages are not necessarily the same as religious marriages. In the United States, anyone who is married in a religious ceremony is considered legally married by the state when the proper documents are filed (usually obtaining a marriage license, waiting a specified period of time, and in some cases taking a blood test). In some European nations, however, a civil marriage ceremony by an officer of the state is required in order for the marriage to be legally recognized, a religious marriage is only for ceremonial purposes.
In the United States, all states have limited Civil Unions to just two people (e.g., the US does not recognize polygamy).
African-American marriages were not legally recognized in the United States until after the Civil War, and mixed race couples were only allowed to marry in all areas of the U.S. after a 1967 decision in the United States Supreme Court. Before that time, 16 states prohibited interracial marriage. Disapproval of interracial relationships was once justified as divinely ordained, however most people today and all of the first world nations accept so-called "mixed" marriages, though there are some extremist groups which still oppose them.
In the United States, most states have enacted laws limiting civil marriages to opposite gender partners, and several have passed constituational amendments. After the 2008 elections, it is likely that another 10-20 states will put limitations of "One man, one woman" into their State constitution. Federal law does not define marriage, as that is one of the Constitutionally derived powers of the States, but the Defense of Marriage Act does prohibit the Federal Government from forcing any state to recognize a so-called "homosexual marriage".
Civil marriages, as opposed to religious marriages which have always existed, are a fairly new concept in human history, developed as governments began to track individuals, (birth and death records), establish and maintain employment taxes (married persons and heads of families are taxed differently from single persons), and provide general order in law about civil matters such as spousal support, limits on spousal abuse, and any form of a welfare or social security system that the government enacted to protect the rights, property and even health of a surviving spouse and children. Prior to that, marriages of any but the highest class were only matters of the various churches, synagogues or other religious institutions.