Depending on the length of the runway, airport is divided into 4 levels. The airport can not accept a plane with higher level. Before you can build the airport of next level, you need to build several airports of previous level. You can check the number of airports you need to build before you can build airport next level in details. Upgrades of airport will affect on all airplanes connected to this airport.
Depending on the length of the runway they needed and flight range, planes is divided into 4 levels. You can buy plane of next level only if you have airport with same level. Plane cannot accept flights longer than its flight range. Profit of flights depends on length of flight and model of plane (passengers, fuel consumption, class). Upgrades of plane will affect only on this plane.
Upgrades improves parameters for planes and airports. You can improve profit from flights or its time, also you can expand amount of hangars in airport.
By accept tasks you can earn gold. But task have expiration date. You must complete the task before this date.
By buying new airports and planes,creating flights and earning gold you're worldwide rating will grow.