One of the most important parts of the aviation industry is air traffic control, or ATC. From the moment a pilot requests clearance to depart to the moment they reach their destination and shut the engines down they are directed by ATC.
Air traffic control is comprised of multiple and specific parts. I will describe how these parts work with each other on a typical flight by an airliner. First, before the pilot departs, they must first get their flight plan cleared. The pilot will ask for clearance from Clearance Delivery. Clearance Delivery will read the pilot their flight plan, after which the pilot will repeat it back to the clearance controller verbatim to ensure that both ATC and the pilot are on the same page.
After clearance has been given, Clearance Delivery will turn the plane over to the airport's ground controllers. The ground controllers will clear the aircraft to taxi to the active runway using specific taxiways so as to ensure no collisions between aircraft and maintain proper flow control. Once the aircraft is approaching the runway, the ground controller will instruct the pilot to switch to the control tower.
The control tower handles only the aircraft on final approach and the aircraft that have just departed. They will not clear an aircraft onto the runway unless there is enough separation between the arriving plane and the one that is about to depart, nor will they clear the aircraft to depart until there is enough separation between the aircraft that is departing and the one that has just departed. When it is safe, the tower will clear the aircraft to take off. Once a predetermined altitude and distance from the airport has been reached, the control tower will inform the pilot that they are leaving the airport's airspace and tell them to switch to the departure controllers.
Departure controllers control the flow of aircraft leaving a certain airspace, not just a specific airport. For example: New York Departure would handle Newark, John F. Kennedy, and LaGuardia (among others) regardless of where they departed. After the aircraft again reaches a predetermined altitude, the departure controllers will then have the pilot switch their radios to 'Center'.
Center is the highest level of ATC. Center monitors the aircraft that are in transit to their destination - usually in the cruising portion of their flight. They maintain radio contact at regular intervals, as well as ensure that the pilot is following the flight plan that they have filed and were cleared for by Clearance Delivery. If not, Center will issue instructions so that the pilot may get back on course. Upon arriving at the point of descent, ATC will monitor the plane until it reaches a certain altitude and will subsequently turn the plane over to the next controller which is 'Approach'.
Approach controllers have the exact opposite role of departure controllers in that they direct the aircraft entering an airspace. They will provide information on weather, traffic conditions, and anything else that will safely guide the aircraft to its destination. They will also tell the pilot to fly at a certain heading, altitude, and speed to set them up for final approach. This is known as vectoring.
Once the approach controller vectors the plane to the final approach heading they will then clear the pilot to approach the airport and instruct the pilot to contact the control tower. The pilot will in turn inform that control tower that they are on final approach for a certain runway. The control tower will make sure that the runway is safe to land on before landing clearance is given. The tower will also inform the pilot of any other aircraft on the ground or in the air that might affect the way the pilot makes decisions regarding the landing.
Once the plane lands, they will exit the runway as quickly as possible to make the runway safe to land for any aircraft that may be on final approach behind it. The control tower will then hand the pilot off to that airport's ground controllers who will then instruct the pilot where to park the aircraft and which taxiways to use. Once the pilot reaches the parking space, or apron, the flight plan is closed be ATC and the flight ends.
In reality the pilot flies the plane and makes sure that it transits safely, but the controllers on the ground are the people who really guide the aircraft to its destination and ensure its safe arrival.