Prenuptial and postnuptial agreements are legal documents that set forth rules married couples must follow upon divorce or death.  These rules pertain to the division of assets and finances.  While these documents are similar, a prenuptial agreement is signed before marriage, and a postnuptial agreement is signed after marriage.
It is imperative to draft either document with the help of attorneys as they must follow specific rules to be upheld in court.  Both must be entered into voluntarily, be in writing, signed and notarized, and be clear, transparent, and fair.