From Citizendium, the Citizens' Compendium
Marriage is a formally recognized long-term relationship between persons. The formal recognition of a marriage is often both legal (different countries and states have laws that give certain rights and responsibilities to married people) as well as religious (many religious texts like the Bible encourage or require marriage). Marriage is usually conducted in some form of ceremony: sometimes in a religious setting like a church, other times as purely a civil event at a registry office.
While the most common form of marriage is between one man and one woman, many other kinds of heterosexual marriage exists, principally polygyny, or a man and several wives. Islam, for example, permits four wives.
In recent years, governments across the (Western) world have had to deal with the question of whether or not same-sex marriage can be allowed. Some have solved this by introducing civil unions or partnerships, while others have used the term marriage.