Air traffic control

Air traffic control (ATC) is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The main job of ATC is to "separate" aircraft to prevent crashes and to make sure aircraft are running on time and as fast as possible. In some countries, ATC may also play a security or defense role (as in the United States), or actually be run entirely by the military (as in Brazil). Air traffic control was first introduced at London's Croydon Airport in 1921. Archie League, who controlled aircraft using colored flags at what is today Lambert-St. Louis International Airport, is often considered the first air traffic controller.
Preventing crashes is done by what is called "separation". This means that the aircraft are kept apart by a minimum distance at all times. Modern aircraft also have collision avoidance systems that see nearby aircraft and warn pilots if other planes are too close. This system is a backup for when air traffic control is not available or if something goes wrong with ATC. Air traffic controllers also can provide information to pilots. But pilots do not have to follow commands of ATC. They can make changes in order to keep plane safe.
Air traffic control service is not provided in uncontrolled airspace. Uncontrolled airspace is usually between ground and 1000 feet except areas near airports. These areas are part of controlled airspace.