The Fast Lane Answer. A ZIP Code is a group of five or nine numbers that are added to a postal address to assist the sorting of mail. A complete, 9-digit ZIP Code, including its additional four digits (called a ZIP+4 Code), is the designation of a delivery route. In other words, regular ZIP Codes indicate which USPS employees will be servicing the address, while the ZIP+4 will tell you which delivery route it will be on.