A ZIP Code is a postal code used by the United States Postal Service (USPS). It is a system of numerical codes that are assigned to each address in the United States. The ZIP Code is composed of five digits, and sometimes includes an additional four digits that further narrow down the location. The first digit of the ZIP Code indicates the region of the country, while the second and third digits indicate a specific area within the region. The fourth and fifth digits indicate a particular post office or delivery area. The additional four digits are optional and are used to identify a specific area within the post office's delivery area.