A marriage can mean different things to different people. In ancient times, for example, a marriage meant a condition in which a woman was given to a man almost as property, and often as part of a political, social, or business arrangement of some sort. For much of human history, marriage has been a permanent institution that, once entered into, cannot be dissolved except by the death of one of the spouses. In the modern world, however, marriage is a vastly different thing. On the up side, marriage is today more of a gathering of equals, rather than the subjugation of one to the other. On the down side, marriage often becomes much more temporary than it has been in years past.
Marriage can be seen in legal terms. In this sense, marriage is a binding contract between the two parties that join together their possessions, income, and lives. Marriage is recognized by the state, and the dissolving of the contract can only happen through the legal process of divorce.
Marriage is also an agreement between the man and woman. Husband and wife take certain vows, to love one another, to cherish one another, and to stay together through sickness and health, for better and for worse. In most cases, this agreement includes sexual faithfulness, and a promise that each person will do what they can to make the other one happy. For some people, this agreement between man and woman takes the form of a covenant between not only the couple, but God as well. Thus, many marriages are performed within the rites of various religious institutions.
A marriage can be viewed from a sociological perspective as well. A marriage is the conduit by which children are born; a marriage provides both a mother and a father for the children. The family unit, the relationship between parents and child, are all based on the marriage relationship.
Certainly, in the modern world, marriage is becoming more complicated. In some areas, same-sex couples are pursuing the right to be married. Of course, a legal recognition of this does not create or eliminate the agreements that couples make between themselves, but these couples desire that legal recognition. In addition, high divorce rates mean that many children will not be raised in a traditional family unit. Here again, the fact that the parents are not married does not mean that they are not a family, but it is changing the way that families are arranged.
Ultimately, marriage is changing, as it has always done. As society changes, the institution of marriage changes. These changes may or may not be a good thing in the end. That notwithstanding, in most, or if not all societies of the world, marriage is intended to bring about the establishing of a happy and responsible family, where the parents and children contribute positively to the society.