Air Traffic Control - communication between Air Traffic Controllers and Pilots, which in IFR (Instrument Flight Rules - eg, the BIG stuff!) keeps the planes seperate and in VFR (the really tiny little planes) just agrees with them and tells them that they can't land.
Pilot: Golf Charlie Foxtrot (G-WACF) Departing Circuit to the North
ATC: Golf Charlie Foxtrot, roger, QNH 1019
Pilot: 1019, Charlie Fox