The grounds for an annulment are significantly narrower than grounds for a divorce. To oversimplify things for the purposes of this post, an annulment basically voids the marriage like it never existed. A divorce simply terminates the marriage. Most of the time, people who are seeking to get out of a marriage are going to go down the divorce path rather than pursue an annulment. However, there are situations in which an annulment may be appropriate.
In oversimplified terms, the grounds for annulment are: if there was no marriage license, if either party was still married to another person, marriage between a brother and sister, marriage between an uncle/niece type relationship, marriage when either party is under 18, lack of consent (because of mental incapacity or infirmity), if either party was a convicted of a felon and the other did not know, if wife was pregnant with another man’s baby and husband did not know or vice versa, if either party had been a prostitute without the other knowing, fraud, duress. If after obtaining knowledge of any of the above, the parties continue to live together and/or act as a married couple, annulment will likely not be an option.
Divorce, on the other hand, can be decreed for a multitude of reasons including that the parties simply had irreconcilable differences and separated (though there are minimum separation requirements prior to obtaining a divorce on this ground). Divorce is typically a longer process and may be more complicated, though not necessarily.
If you are unhappy in your marriage and are looking to terminate it, consult with an attorney to determine the best possible process for you to pursue.