At a major controlled airspace like ORD, they will have:

- approach controllers
- departure controllers
- tower controllers
- ground controllers
- then the individual air carriers will have controllers for their section of the airport (ie, their assigned gates)

In the air, there are "en route" controllers.

So for a plane coming into ORD, the en route center will hand the plane off to ORD approach, when the plane gets close enough, it is handed off to ORD tower, as soon as it is down on the active runway, the plane contacts ground to find out how they're getting off the active runway, and when the plane is turning off the taxiway near the terminal, they contact their company gate controller. There are lots of frequency changes for the pilots to contend with. Radio procedure needs to be crisp and tight so that they can get all the instructions transmitted and verified in the time they have to work with.

IMO, the most stressful and overworked controllers are the ground controllers. They control everything that moves on the ground at the airport - planes, tugs, fire trucks, rescue vehicles, inspection vehicles, etc. They all have to be controlled where they go, how they get there, how they get out of there, where they go as they're getting off the tarmac, etc. There's more potential for an accident on the ground than once the aircraft are in the air, believe it or not.