Marriage is a social union or legal contract between people that creates kinship
Kinship is a relationship between any entities that share a genealogical origin, through either biological, cultural, or historical descent. And descent groups, lineages, etc. are treated in their own subsections....

. It is an institution
An institution is any structure or mechanism of social order and cooperation governing the behavior of a set of individuals within a given human community...

 in which interpersonal relationships, usually intimate and sexual, are acknowledged in a variety of ways, depending on the culture
Culture is a term that has many different inter-related meanings. For example, in 1952, Alfred Kroeber and Clyde Kluckhohn compiled a list of 164 definitions of "culture" in Culture: A Critical Review of Concepts and Definitions...

 or subculture in which it is found. Such a union, often formalized via a wedding
A wedding is the ceremony in which two people are united in marriage or a similar institution. Wedding traditions and customs vary greatly between cultures, ethnic groups, religions, countries, and social classes...

 ceremony, may also be called matrimony.

People marry for many reasons, including one or more of the following: legal, social, libidinal
Libido refers to a person's sex drive or desire for sexual activity. The desire for sex is an aspect of a person's sexuality, but varies enormously from one person to another, and it also varies depending on circumstances at a particular time. A person who has extremely frequent or a suddenly...

, emotional, economic, spiritual, and religious.

Marriage is unusual in that the more I see of it, the less I believe in it.

Jacob M. Appel, The Mistress of Wholesome|The Mistress of Wholesome

A husband is what is left of a lover, after the nerve has been extracted.

Helen Rowland, A Guide to Men

Some marriages give bachelors a master's degree.

Leonid S. Sukhorukov|Leonid S. Sukhorukov, All About Everything

Marge: Homer, is this the way you pictured married life? Homer: Yup, pretty much. Except we drove around in a van solving mysteries.

The Simpsons and others.

A marriage so free, so spontaneous, that it would allow of wide excursions of the pair from each other, in common or even in separate objects of work and interest, and yet would hold them all the time in the bond of absolute sympathy, would by its very freedom be all the more poignantly attractive, and by its very scope and breadth all the richer and more vital -- would be in a sense indestructible.

Edward Carpenter, Love's Coming of Age

Marriage is wonderful when it lasts forever, and I envy the old couples in When Harry Met Sally who reminisce tearfully about the day they met 50 years before. I no longer believe, however, that a marriage is a failure if it doesn't last forever. It may be a tragedy, but it is not necessarily a failure. And when a marriage does last forever with love alive, it is a miracle.

Peggy O'Mara, Mothering, Fall 1989

Marriage, n. A community consisting of a master, a mistress, and two slaves, making in all, two.

Ambrose Bierce, The Devil's Dictionary

A bad marriage is like an electrical thrilling machine: it makes you dance, but you can't let go.

Ambrose Bierce, A Cynic Looks at Life, 1912

There's nothing a woman hates more than her fiance's best friend. He knows all the secrets she's going to spend the rest of her life trying to find out.

Jeff Douglas line in the musical Brigadoon