Marriage Annulment is a legal endowment for proclaiming a marriage null and void marriage. In India sometimes due to force or undue power or fraud in unavoidable conditions marriages happens. After some year spouse realize it was mistake or try to find out ways to get rid of the forceful relationship. Most of the time parties want to avoid stigma of divorcee because they feel that the relationship in which they were in was not at all marriage and annulment of marriage is way to avoid burden of Divorce. In simple term if you married with person but it was never consummated.
Annulment of marriage is tool to declare nullity of marriage and declare it null and void marriage. It is indirectly called cancellation of marriage.
Annulment law is not easy to understand. You have to give the concert proof and only in specific circumstances wherein court applies strict terminology and declare annulment.