Annulments and divorces are similar in the sense that they make a determination about marital status but different in that divorce ends an existing, valid marriage, whereas an annulment declares that a marriage never actually existed.
In Illinois, you can request a judgement of invalidity, which is a final order saying that your marriage was never valid. It is rarely granted and the court has set strict guidelines on what circumstances would allow a couple to get an annulment.
Under 750 ILCS 5/220) Illinois Marriage and Dissolution of Marriage Act there are four circumstances that would allow you to obtain an annulment (Declaration of Invalidity):
1. At the time of the marriage ceremony, one of the parties:
Lacked mental capacity to consent to the marriage because of:
Mental incapacity or infirmity (psychosis, dementia, or mental retardation, for example) or The influence of alcohol or drugs
One party does not possess the physical ability to consummate the marriage by sexual intercourse, and the other party did not know about the incapacity at the time of the marriage
One or both parties were underage and did not have the consent of their parents or guardians, or judicial approval.
The marriage is prohibited. Examples of prohibited marriage include:
One party was already married at the time of the marriage ceremony or The parties are close blood relatives (siblings, uncle and niece, aunt and nephew, first cousins, etc.)
If you are going to try and obtain an annulment, you must be able to prove one of these factors to the courts. However, if you want an annulment for religious reasons, you should consult with you priest, minister, or rabbi as well as civil and religious annulments are different.