Whenever I speak about my job with people, even out of the aviation business, they dont always know what ATC is. In Germany most people think, an air traffic controller is the person, guiding the aircraft into the parking stand. "Well, he is called a marshaller", I always explain. "I guide the pilots to the airport and keep them clear of other air traffic". "Oh, so you are the man in the control tower?", is what i hear next. Well..., almost!
The main task of Air Traffic Control is, to ensure a safe, orderly and expeditious flow of air traffic. Therefore, every country has air navigation service providers (ANSP's), which establish control towers at airports and Air Traffic Control Centers (ATCC's) for the airspace between and above airports. This huge airspace is divided in so-called "sectors", having individual lateral and vertical dimensions. So on a flight from Frankfurt to London for example, the flightcrew will talk to ATCO's on two towers as well as many different control sectors, each staffed with two controllers.
It will always be ensured, that they will stay well clear of other airplanes and fly the most possible direct and efficient way to their destination.