In the aviation world, every location has two identities.
- IATA (3 letters): Used by airlines, travel agencies, and baggage tags (e.g., MAD, JFK, LHR). It is what you see on your ticket.
- ICAO (4 letters): Used by pilots, air traffic control, and flight planning (e.g., LEMD, KJFK, EGLL). It tells you about the region and country.
If you are a traveler, stick to IATA. If you are a pilot or flight simmer, use ICAO.