A Divorce and an Annulment have the same end result, they both dissolve the marriage. However, in a divorce situation, the former spouses are still recognized as having been married, while an annulment treats the marriage as though it never existed, it voids the original marriage contract, ie. the marital union was invalid or void from the beginning.
The choice of whether to get a divorce or annulment is personal, one of the differences is a matter of labeling, a spouse is technically unmarried or single if an annulment is obtained, while the spouse is divorced after obtaining a divorce decree. Another important difference is that a divorce can be "no-fault" where one spouse does not have to allege fault on the part of the other. An annulment application on the other hand must allege fault on the part of the Defendant spouse, such as lack of physical or mental capacity to enter the marriage, bigamy, or fraud, duress or undue influence brought to bear by the Defendant spouse on the other to enter the marriage.