Controller is an object that performs AI-routines. In other words a controller is an instance of the AI. Controller stores the current main task, active enroute tasks, and behavior options. Controllers also can perform commands. Controllers exist at both a group and unit level. However only planes and helicopters can be controlled individually at a unit level. Some functions can only work for Unit Controllers.
Different tasks, options, and commands are available for the different group types. (Plane, Helicopter, Ground Unit, and Ship). See the linked articles on these subjects for more information.