Marriage is a traditional way of living together by a man and a woman. This is an accepted way of living of people among a large society with discipline and regulations, so that interests of people do not harm and cross each other in a destructive manner.
Marriage is a life long bond and close association for the two being wedded, to take care of each others' needs in a constructive way. Also marriage is a association between a man and woman to satisfy their personal, emotional, social and biological (including sexual) needs. It is a live-in relationship with associated positive and negative points. Marriage is a social custom through which a person progresses his/her clan ahead in future.
Marriage is an institution to build a new family in a new home. Ultimately that leads to building up of the society and nation. One way to look at a marriage is that a country is made of families at a gross level. Families are created through marriages. Marriage is a merger of two different families and clans.
Marriage brings with it a responsibility of taking care of another person in a positive way. Marriage starts with a lot of celebration, ritual, religious or other tradition. This is only the beginning of an institution of marriage, and it continues life long.