A prenuptial agreement is prepared prior to getting married. It is appropriate to have it drafted when you want to put an agreement in place and do not want the court to decide what happens to your property, assets, and debts should the unfortunate circumstance of a divorce occur. Although no one foresees to get divorced when they get engaged, it is similar to having insurance; the risk of needing it is always there though the risk may currently be slim.
A postnuptial agreement is prepared after you get married. It is an agreement that will decide what happens to your property, assets, and debts. Some people have one drafted when they did not have time to have a prenuptial agreement prepared. With both types of agreements you should consult an attorney to make sure you have all of the elements necessary for your agreement to be enforceable