A prenuptial agreement protects those individuals who are entering a marriage who are bringing significant assets into the marriage and wish to protect those assets from the equitable distribution process if a divorce occurs later on.
A postnuptial agreement is entered into after a couple is married, often for the same or similar reasons that the prenuptial agreement. The only main difference is how they tend to take into account things that are known rather than assumed during a prenuptial agreement, like property and children.