Divorce is the complete and total dissolution of a marriage that pre-existed between the spouses. Therefore once it is granted the couple is no longer deemed to be married in the eyes of the law and thus are able to remarry. Consequently they are no longer bound by the obligation of fidelity and cohabitation.
Separation is the physical separation of the spouses without dissolving the union of matrimony. The spouses will also split their assets and decide on their matrimonial home. Other issues concerning the children of the marriage like custody and access rights are agreed upon. However in the eyes of the law the couple are still considered to be married and thus cannot seek to remarry once they are only separated.
Annulment on the other hand states that the marriage never existed due to a vice when the couple got married to each other. Once it is granted the couple are free to remarry once again since in the eyes of the law they are deemed to never have wed.
