Controller keyboards were introduced shortly after the invention of MIDI (Musical Instrument Digital Interface) which is a system that allows electronic musical instruments to transfer digital data between themselves and effectively "communicate" with each other.
A controller keyboard's physical appearance is little different from a standard MIDI keyboard however it does not contain internal sound patches as it is designed to drive or 'control' other MIDI keyboards, modules or devices. Soon after MIDI keyboards were introduced manufacturers also designed and produced MIDI modules which were the sound patches, processors, electronic memory chips and PCBs (Printed Circuit Boards) housed in a MIDI module, but without the actual keyboard.
This was done so that the master or 'controller' keyboard could connect to the module via MIDI cables and use its own keyboard to play back the sounds found within the module. Controller keyboards can connect to several MIDI sound modules simultaneously and the keyboard keys can be zoned into multiple sections which can be assigned to specific MIDI modules. MIDI patch bays can simplify the connectivity between controller keyboards and modules.