ICAO airport identifiers are unique four-character codes designating airports around the world. Defined by the International Civil Aviation Organization, the codes begin with one or two characters designating the country. They differ from International Air Transport Association, or IATA codes, which contain three letters.
Most Americans are more familiar with IATA codes as those codes, agreed upon by the members of a aviation trade organization, appear on tickets, baggage claim checks, and flight arrival and departure terminals. However, as air traffic control is an international affair, the four-letter codes of the ICAO, an agency of the United Nations, are those used by air traffic controllers and flight planners.
ICAO codes for most airports in the United States are simply the IATA code prepended with a "K," which is the ICAO country code for the United States. For example, Los Angeles International Airport, whose IATA code is LAX, and John F. Kennedy Airport, which has the IATA code JFK, have ICAO designations as KLAX and KFJK, respectively. So while an airline may ticket shows a flight from LAX to JFK, the flight plan filed shows the flight as KLAX to KJFK.