In 1963, the United States Postal Service established the ZIP -- Zoning Improvement Plan -- Code system and assigned a five-digit code to every mailing address in the country. The system allowed postal workers to sort the increasing mail volume faster. In 1981, the ZIP+4 code extension was instituted to further narrow down the geographic area for mail delivery. The USPS website or any USPS location can tell you the four-digit ZIP Code extension for a specific address.
Visit the USPS website and click on the link titled "Look Up a ZIP Code." Enter the mailing address in the appropriate boxes and click "Find." For example, enter 1600 Pennsylvania Ave NW, Washington, DC if you are looking up the ZIP Code+4 Extension for the White House. In the search results, you will see the complete nine-digit extension is 20500-0003.
The first five digits are the standard ZIP Code, which identifies the metropolitan delivery area -- such as Orlando or Tampa. The last four new digits narrowed down the delivery area further, with two digits representing the neighborhood and two digits identifying the street block or building. The post office does not require senders to use the full nine digit ZIP Code, but it can reduce the number of times your mail is handled and decrease the chance of a delivery error.