The International Civil Aviation Organization, or ICAO, is an agency directed by the United Nations which helps to streamline interaction among the aviation industry of the UN’s member countries.
It is similar to the International Air Transport Association (IATA), which also encourages standardization between airlines and airports of competing brands and different countries; however, the IATA is a trade organization, whereas the ICAO is essentially a government entity, being under the United Nations. It is headquartered in Montreal, Canada, as is the IATA.
The International Civil Aviation Organization contains, as members, 191 member states (190 + Cook Islands), which is just about all of the United Nations’ countries. Working with these member states and the various aviation industry organizations and entities within them, the ICAO promotes consensus-driven best practices and implementations, industry-wide.
Again, similar to the IATA, the International Civil Aviation Organization utilizes their own airline codes and airport codes. The ICAO’s airline codes are three digits, and their airport codes are four digits: “AAL” for American Airlines and “WSSS” for Singapore’s Changi Airport. The IATA’s codes remain the more popular.