DEFINITION:

It comprises:

• Air Traffic Control: the process by which aircraft are safely separated in the sky as they fly and at the airports where they land and take off again.
• Air Traffic Flow Management: an activity that is done before flights take place. Any aircraft using air traffic control, from a business airplane to an airliner, files a flight plan and sends it to a central repository. 
• Aeronautical Information Services: they are responsible for the compilation and distribution of all aeronautical information necessary to airspace users (safety, navigation, technical, administrative or legal matters and their updates).

(Source: Eurocotrol: http://www.eurocontrol.int/articles/what-air-traffic-management and  PAE own elaboration)