A controller card is a hardware component that works as an interface between the motherboard and other computer components. It is a built-in component found in most computers. A controller card is installed in the computer's PCI slot and is integrated with the motherboard.
A controller card may also be known simply as a controller.
Techopedia explains Controller Card
A controller card can be an electronic integrated chip, an expansion card or a stand-alone device that works as a crossing point with peripheral devices. The basic purpose of the controller card is to provide additional features to the motherboard in managing all the connected internal and external devices.
Typically, all devices require a controller card to work efficiently. In the event of an absence of an internal controller slot, an external controller card is installed. A controller card may include a memory controller, storage controller, input device controller and many others, each of which serves its distinct device type.