An annulment is often misconstrued as a divorce, Catholic-style. Nothing could be further from the truth.
An annulment is a declaration by the Catholic Church that, after careful investigation, flaws were discovered in the relationship that prevented it from becoming a marriage. These flaws have to exist prior to consent in a way that would impede true consent and result in the invalidity of the marriage. An annulment has no bearing in civil law. This is why a civil divorce precedes the commencement of an annulment.
Canon Law dictates the area that would impede a valid marriage. First, some impediments, like a previous bond of marriage, religious vows or holy orders, non-age (not of canonical age) or close blood relationship (consanguinity) prevent the sacrament from occurring.
Some can invalidate the consent, like lack of mental competence, grave lack of discretionary judgment, or serious psychological problems. Second, homosexuality, alcohol or drug addiction/abuse, gross immaturity, etc., can also affect the validity. Third would be that at the time of the wedding, the bride or groom held an intention contrary to the essential understanding of Christian marriage, such as refusing to have children.
The annulment is processed through a Church court known as a tribunal. The petitioner is assigned an advocate. Witnesses are required because marriage is a public event. A defender of the bond is employed that defends the bond of marriage. After the witnesses and petitioner are interviewed and an external forum is convened, which determines that there was in fact something that impeded true consent of the marriage, then an annulment is granted. The annulment is automatically sent to the next higher ecclesiastical court to make sure the process was done properly and to hopefully ratify the judicial decision.
A documentary annulment process is different from the formal annulment process. The first kind is a process of paperwork that applies when the marriage was invalid from the start because of a lack of canonical form is a process of paperwork that applies when the marriage was invalid from the start because of a lack of canonical form. To be valid, Catholic marriage must be between two consenting adults who state their intentions before two witnesses and a valid Catholic minister inside the Church. Lack of canonical form means one of these things was missing.
If a baptized Catholic marries outside the Church without a dispensation, the marriage is invalid. To obtain the declaration of nullity, a priest or deacon has to obtain baptismal certificates and the marriage certificate, and the petitioner has to fill out a questionnaire along with two witnesses who will testify that the couple never had their marriage convalidated in the Catholic Church.
The paperwork is sent into the chancery office, and a decree of nullity is given.