Most Air Traffic Controllers work for the government and control aircraft in different regimes of flight. They have full governmental authority over the airspace they control and while final authority over any particular flight always rides with the Captain, there must be a valid reason for any deviation from controller’s instructions.
Ramp Control – airline controlled and do not have regulatory authority.
Ground Control – Once clear of the ramp the airliner will be handed to ground control for directions to the runway.
Tower Control – From there the flight, is controlled by one or more tower controllers who handles the flight for takeoff . He will handle the aircraft throughout the initial stages of flight.
Departure Control – Once in the air the airliner will be handled by one or more local controllers called departure control. They separate the flights from the airport and head them in the direction they need to go. Occasionally, in very large areas the local control may be combined with an area control into one terminal control center.
Enroute Control – Once the aircraft has left the control of the departure controller it enters enroute airspace controlled by ARTCC (air route traffic control centers) in the USA (i.e. Atlanta Center in Southeast USA), Control Zones, or Area Control Centers internationally (i.e. Naha Control over Taiwan). Some centers have ICAO designations for controlling traffic over the Oceans of the world (i.e. Oakland Oceanic is the world’s largest covering nearly 19 million square miles.)
Approach Control - Once entering the local area for the arrival airport the airliner once again enters the local control at the destination airport. Co-located with the departure control the arrival control (or approach control) aligns the incoming aircraft for the runways at the airports under their control. This could be one runway at one airport, multiple runways at one airport (possibly control by multiple controllers) or multiple runways at different airports. It simply depends on the amount of traffic.
Tower Control – handling the traffic from the approach controllers the airplane is given final instructions necessary for landing. Usually stretching for only a few miles, they control the airport traffic area. They may alter the final runway if conditions require including LASHO instructions (land and hold short operations), planned turn off points, final speed instructions or side step.
Ground Control – Once the airplane has landed, they will once again be handed over to the ground controller who will either direct the airplane to the gate or appropriate ramp control.
Dispatchers are licensed individuals who usually work directly for an airline. In Part 121 aviation in the US and similar regulatory agencies in other parts of the world, pilots and dispatchers are required to launch an airline flight. The Captain, in coordination with the dispatcher, releases the flight. In operational practice at major airlines, the dispatchers will gather the aircraft data from maintenance on the scheduled aircraft for the flight he is dispatching. In review of this data, the dispatcher will determine whether the aircraft is fit for the particular flight in question (MEL for an ETOPS flight). Once that is accomplished (or simultaneously) the dispatcher will also get met and route data for the flight. In most cases, there are canned flight plans that provide the route data, but for Oceanic or long haul domestic flights the actual routes change daily (NAT Tracks etc).
Once the dispatcher has this, he plugs the met data along with the route data into a program to calculate the fuel requirements. Finally, he will give a dispatch envelope (or computer file in some cases) along with any required plotting charts to the crew. The crew will review all the data presented, from forecast weather, flight planned altitude, fuel, enroute wx, aircraft status, track designations, enroute alternates, ETOPS plans (90, 120, 180 etc), HF frequencies, Data Link status, crew compliment and anything else they may need or want (i.e. additional weather for another city etc)
If the captain desires a change, he must get agreement from the dispatcher and in some cases, this may not be forthcoming. The dispatcher will state why not, the captain will argue his reasons, and in nearly all cases for major airlines, they will agree on the safest result. If the captain cannot agree then he will not take the flight or the dispatcher will not release it.
So controllers or air traffic controller work for the government and control all airplanes operating in a system. Dispatchers work for an airline or service company and release individual flights in coordination with the Captain of that flight. Controllers guide airplanes through the air and on the ground while dispatchers make sure any individual commercial flight has been properly released.