Divorce and/or judicial separation means there was a valid marriage, whereas an annulment can occur when there is a defect or problem with the marriage itself that exists at the time of the marriage. Generally, children of an annulled marriage are still considered legitimate.
Grounds for Annulment
In order to be annulled, your marriage must be either void or voidable. A marriage is void or voidable usually when the parties have been legally incapable of marriage. For example, where persons are too closely related or lack the mental capacity to understand the nature of the marital commitment, or where there has not been voluntary consent to the marriage or when the marriage has not been consummated. Legal advice will be necessary to determine whether your marriage meets these or other grounds for annulment.