Basic configuration of the controlled airspaces and their respective air traffic control offices.
The TWR (aerodrome control tower) usually controls traffic that occurs inside the ATZ (aerodrome traffic zone) of the aerodrome where this TWR is installed at. The APP (approach control) usually controls traffic that occurs inside the CTR (control zone) but outside of the ATZs. The APP also controls traffic ocurring inside the TMA (terminal control area). The ACC (area control centre) usually controls traffic occurring inside any AWY (airway). In this image, the surrounding airspace (outside of the ATZs, CTRs, TMAs and AWY) is a non-controlled airspace designated a Flight Information Region (FIR). When an aircraft is flying into a FIR, no Air Traffic Control (ATC) office will provide any traffic control to this aircraft's pilot(s), however the ACC will provide it the Flight Information Service (FIS) and the Alerting Service (ALRS).
<a href="//commons.wikimedia.org/wiki/User:Sampayu" title="User:Sampayu">Yuri Ribeiro Sucupira</a>
<span class="int-own-work" lang="en">Own work</span>