Divorce, also known as dissolution of marriage, refers to the legal process of ending a marriage.
Annulment refers to invalidating a marriage through a court procedure that requires showing that the marriage was never valid.
Separation means no longer living together. It is usually voluntary and informal.
Divorce, dissolution, annulment and separation all are part of Family Law. Family law and procedure varies from state to state in the United States.
Use: Divorce and annulment are formal procedures available to a couple to permanently end their marriage and change their marital status. Separation is less formal and is generally used to temporarily change a couple's marital status.