A postal money order is a payment order issued by the United States Postal Service. They are very similar to the money orders issued by banks.
USPS money orders can be purchased and redeemed at almost any USPS retail counter. Many banks and financial institutions also accept them, and they can be signed over to another party as payment. They are a popular form of payment, as they never expire and come with a receipt that potentially allows them to be replaced if lost or stolen. The fee is also usually low relative to what banks charge for money orders and cashier's checks.