When you get divorced, it ends your marriage. An annulment, on the other hand, treats a marriage as though it never existed. The two aren't interchangeable, and an annulment is more difficult to get than a divorce.
Although there's less stigma attached to divorce now than there used to be, some people still view it as socially unacceptable and try to pursue an annulment instead. Other people might seek an annulment for religious reasons.
A marriage can be annulled only when the law concludes that your marriage was "void" or "voidable" based on the circumstances surrounding the marriage.
There's a major distinction between "void" and "voidable." A void marriage automatically qualifies for an annulment, because it's based on an illegal act. In a voidable marriage, however, legal reasons for an annulment may exist, but they won't invalidate the marriage unless one of the spouses requests the annulment.
As indicated above, a void marriage wasn't legal to begin with. Two of the most common underlying reasons for considering a marriage void are the illegal acts of "bigamy" and "incest". Bigamy happens when someone enters a marriage while already married to another person. An incestuous marriage occurs when the spouses are close family members. The definition of "close family member" varies from state to state. Some are obvious, like brother and sister, but some states also ban marriage between first cousins.
Some of the more common scenarios that could make a marriage voidable are:
Regarding voidable marriages, it's important to note that if you ratified (approved) the marriage after discovering the circumstances that could make it invalid, it's not likely a court will grant an annulment. Continuing to live together as a married couple after learning about the condition that could potentially invalidate the marriage is a typical example of ratification.
The most obvious effect of an annulment is that it renders the marriage null and void. But there are other possible consequences. For example, it could impact a spouse's ability to get support (alimony) from the other spouse. Likewise, it might affect a spouse's rights to property acquired during the marriage. So you should definitely consult with a family law attorney before making any decisions regarding annulment.
Most annulments happen shortly after a marriage, although there can be exceptions. Some states won't allow an application for an annulment after a certain time. Learn more about your state's annulment laws.