Controller is a device that controls the transfer of data from a computer to a peripheral device and vice versa. For example, disk drives, display screens,keyboards, and printers all require controllers.
In personal computers, the controllers are often single chips. When you purchase a computer, it comes with all the necessary controllers for standard components, such as the display screen, keyboard, and disk drives. If you attach additional devices, however, you may need to insert new controllers that come on expansion boards. Controllers must be designed to communicate with the computer's expansion bus. There are three standard bus architectures for PCs.The AT bus, PCI (Peripheral Component Interconnect), and SCSI. When you purchase a controller, therefore, you must ensure that it conforms to the bus architecture that your computer uses.
In control theory, a controller is a device, possibly in the form of a chip, analogue electronics, or computer, which monitors and physically alters the operating conditions of a given dynamical system.