A Cab is the (typically handheld) device that is used to control a train. A basic cab has controls for speed, direction and a few other functions. More sophisticated cabs have more functions including the capability to create consists and program decoders. CAB is the preferred NMRA definition for the control device. The term is derived from the fact that the operator has all the controls the engineer has in the cab.
Cab Control is an old analog method, which involved a lot of electrical wiring and switches to connect the cab to the train the user was controlling.