In both the new and old systems of air traffic control, airspace is divided into controlled and uncontrolled airspace. Controlled airspace is that part of the sky where traffic density is so high that strict control of aircraft movements is required - this is the part that most concerns air traffic controllers. You can think of controlled airspace as a bit like a walkway that leads up a stairway, along a raised corridor and down a stairway on the other side. The stairways represent the airspace above and adjacent to an airport - it is in this space that large commercial airlines either depart or approach the runway. For the entire trip, all aircraft are monitored and controlled by air traffic control.
Uncontrolled airspace is the part of the sky that is outside controlled airspace.
Collision avoidance in uncontrolled airspace relies largely on the wits of the pilot and on agreed ways of separating traffic, such as by flying at different altitudes depending on the direction of flight. Recent trials of new procedures for this airspace have been the subject of some controversy. Another critical part of air traffic control is what happens on the ground. This is the domain of ground traffic controllers, who direct aircraft as they taxi about the airport. Large aircraft may look graceful in the sky but on the ground they are awkward and unwieldy, and they need to be directed carefully to avoid collisions.