An annulment is like the eraser on the end of a pencil. If you make a mistake, you can turn the pencil around and rub your writing away; it's as though it never existed. This is in contrast to a divorce, which is more like making a big "X" through your writing; it's still there, but you've crossed it out. When your marriage is annulled, the court rules that it never happened, whereas if you divorce, the court ends an existing marriage.
The majority of marriages cannot be annulled. Special grounds must exist -- and few marriages qualify. Although grounds vary somewhat from state to state, they typically include situations where you or your spouse were already married at the time of your wedding, or the marriage took place as the result of fraud or duress. Underage marriages can also be annulled, as can those where a spouse is impotent. If you think you might have grounds, speak with a local lawyer.
Kids, Property and Support
Because an annulment rules that your marriage never existed, this can affect certain aspects of ending it. For example, in Ohio, you have no right to alimony if you have your marriage annulled. If you and your spouse have a child, however, annulment generally does not affect issues of child support or custody. If you acquired any property during your marriage, distribution depends a great deal on where you live. Some states will not divide marital property in an annulment, particularly if the marriage was annulled due to bigamy. Others, such as California, will do so.