The controller card, or simply "controller," is a piece of hardware that acts as the interface between the motherboard and the other components of the computer. For example, hard drives, optical drives, printers, keyboards, and mice all require controllers to work. Most computers have all the necessary controllers built in the motherboard as chips, not full-sized cards. However, if you add additional components such as a SCSI hard drive, you may need to add a controller card as well. Controller cards are typically installed in one of the computer's PCI slots.