The Catholic Church understands marriage to be a covenant established between a man and woman in which the essential obligations are unity and indissolubility. For those who have been baptized, a valid marriageis also a sacrament. Not all marriages are a sacrament, but all marriages (Catholic, Protestant, Jewish, nonbeliever, etc.) are presumed to be valid. The good of all concerned (spouses, children,in-laws, society, the Church, etc.) demands this presumption.
However, in every presumption the opposite may be true. If sufficient evidence can be shown that an essential element was lacking at the very root of the marriage then the validity of the marriage becomes questionable and is open for investigation. This is when a marriage invalidity case (or the marriage annulment process ) takes place within a Catholic Tribunal.
For more information please click on this link: http://www.catholicnh.org/family/single/annulment/