Robot Controller
The robot controller is the cabinet and software that drives the motors, runs the program, and handles I/O and safety, e.g. FANUC R-30iB, KUKA KRC4, ABB IRC5.
The robot controller is the computer-and-drive cabinet that executes the robot program, commands each axis's servo motor, and manages I/O, communication, and safety functions. Each manufacturer uses its own controller generation (e.g. FANUC R-30iB, KUKA KRC4/KRC5, ABB IRC5/OmniCore).
The controller, not just the arm, defines what a robot can do: supported fieldbuses, software options, safety features, and teach pendant. A robot and its controller are a matched pair — they are sold and refurbished together.
When buying a used robot, check the controller generation, installed software options, and whether the teach pendant and cables are included. An obsolete controller can be the hidden cost in an otherwise cheap arm.