According to the USA Federal Aviation Administration (FAA): "Prohibited areas contain airspace of defined dimensions identified by an area on the surface of the earth within which the flight of aircraft is prohibited. Such areas are established for security or other reasons associated with the national welfare. These areas are published in the Federal Register and are depicted on aeronautical charts."
Some prohibited airspace may be added or expanded via NOTAMs. For example, Prohibited Area 40 (P-40) is often expanded when the President of the United States visits Camp David in Maryland, while normally the airspace surrounding P-40 is restricted, not prohibited.
Violating prohibited airspace established for national security purposes may result in military interception and/or the possibility of an attack upon the violating aircraft, as in the case of Korean Airlines Flight 007. Aircraft violating or about to violate prohibited airspace will often be warned beforehand on 121.5 MHz, the emergency frequency for aircraft.