Marriage is a form of interpersonal relationship; a kind of social contract. The word "marriage" by itself encompasses a broad range of meanings, but it is often short for Christian marriage or holy matrimony, which is ordained by God and considered a sacrament by Roman Catholics (but not most Protestant denominations). In a few times and places, other forms existed, such as polygamy (multiple spouses) and homosexual marriage (two people of the same sex).
In most cultures, marriage is regarded as a life-long commitment to another person, under any circumstances. Frequently this involves a public commitment ceremony (called a wedding) to each other and, if the participants are Christian, before God. Unfortunately, sometimes people do not or cannot maintain that commitment, and the marriage ends in divorce.
In most countries, married couples are treated as a single economic entity, and when one partner dies, the other may inherit all of their property in a simpler system than would be required without the marriage.
African-Americans were not allowed to marry in all areas of the United States until after the Civil War, and mixed race couples were only allowed to marry in all areas of the US after a 1967 decision in the United States Supreme Court. Before that time, 16 states prohibited interracial marriage. Disapproval of interracial relationships was long justified as divinely ordained and 1968 public opinion polls indicated that 72% of Americans disapproved of interracial marriages.
Same-sex couples are not allowed to marry in many areas of the world, although this restriction was lifted in April 2001 by Holland and January 2003 by Belgium, which some conservative commentators have claimed is causing the destruction of traditional marriage in those countries. By the end of 2004, same-sex marriage was legal in most Canadian provinces and in the Yukon and is now enshrined in Canadian federal law C-38.  Same-sex marriage is also legal in Spain and South Africa.
Marriage in the Bible
The Old Testament clearly allows polygyny. (Gen. 4:19, Gen. 16:1-4, Gen. 26:34, Gen. 32:32, Exodus 21:10, Dt. 21:15, Judges 8:30, I Sam 1:1-2, II Sam 12:7-8, I Kings 11:2-3, I Chron. 4:5, II Chron. 11:21, II Chron. 13:21, II Chron 24:3.) This is less common, but never outlawed, in the New Testament.
The Old Testament allows for the purchase of wives (Gen. 29:18, I Sam. 18:25-27).
The clearest description of marriage in the Bible comes from I Corinthians 7:1-16:
Now concerning the things whereof ye wrote unto me: It is good for a man not to touch a woman. Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
Let the husband render unto the wife due benevolence: and likewise also the wife unto the husband. The wife hath not power of her own body, but the husband: and likewise also the husband hath not power of his own body, but the wife. Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.
But I speak this by permission, and not of commandment. For I would that all men were even as I myself. But every man hath his proper gift of God, one after this manner, and another after that.
I say therefore to the unmarried and widows, It is good for them if they abide even as I. But if they cannot contain, let them marry: for it is better to marry than to burn.
And unto the married I command, yet not I, but the Lord, Let not the wife depart from her husband: But and if she depart, let her remain unmarried, or be reconciled to her husband: and let not the husband put away his wife.
But to the rest speak I, not the Lord: If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away. And the woman which hath an husband that believeth not, and if he be pleased to dwell with her, let her not leave him. For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy. But if the unbelieving depart, let him depart. A brother or a sister is not under bondage in such cases: but God hath called us to peace. For what knowest thou, O wife, whether thou shalt save thy husband? or how knowest thou, O man, whether thou shalt save thy wife?
In most branches of Christianity, marriage is a life-long religious union between a man and a woman. However, many Christian groups acknowledge that celibacy is more holy than marriage, and some, such as the Shakers, have gone so far as to prohibit marriage.
Historically, Mormonism has practiced polygyny (more than one wife). It stopped being a part of mainstream Mormonism in 1890, but some fringe groups still practice it.
Islam allows Muslim men to have up to four wives. The Qu'ran says(4:3): "Marry women of your choice, two, or three, or four; but if you fear that you shall not be able to deal justly (with them), then only one." 
Before Islam, it was common for a man to have many wives. The limit of four imposed by Islam was a way of controlling the situation. A Muslim may marry up to four women only on condition he deals justly with all of them. For example, he must provide each with separate living accommodation. Since this is difficult, most Muslim men in practice do not have more than one wife.
The reason for allowing multiple wives was for the welfare of widows and orphans. Muslim men are required to be financially responsible for the members of their family, and this was a way of ensuring that there was somebody to take responsibility for widows' and orphans' welfare.
It also allowed for a man who wished to father children to take another wife if his first wife was barren, and allowing the first wife to choose whether to continue in the marriage. A Muslim man is not allowed to marry another woman without the knowledge and consent of his wife, and the wife has the right to ask for divorce if she does not agree.
Civil marriage is a legal contract. In the state constitutions of almost all states, this contract must be between one man and one woman. It gives each member certain rights and obligations. 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.
Although not going as far as Holland and Belgium in making same-sex marriage legal, the Scandinavian nations and the United Kingdom allow same-sex couples to gain benefits by registering a legal commitment, although they acquire fewer advantages than opposite-gender couples receive automatically upon marriage. Germany, France, Andorra, New Zealandand several parts of Australia. Different names are given to civil unions in different legal jurisdictions. This include civil pacts, civil partnerships, life partnerships, registered partnerships, domestic partnerships, significant relationships, reciprocal beneficiary relationships, and stable unions.
In the U.S. there are over a thousand federal benefits to marriage but none to same-sex civil partnerships. Couples registering same-sex relationships receive some state benefits in California, Connecticut, Massachusetts, New Jersey and Vermont. Rights accruing from same-sex marriages contracted in other states are respected in Rhode Island, and some restricted benefits are available to same-sex partners in Hawaii and Maine.
- The American Heritage dictionary includes legal marriage between a man and a woman, common-law marriage, and same-sex marriage
- The Marriage Service, The Church of England, Retrieved 20 March 2007
- Virginia v Loving