A peripheral is a type of computer hardware that is added to the computer, in order to expand its abilities. More specifically the term is used to describe those devices that are optional in nature, as opposed to hardware that is either demanded, or always required in principle. The term also tends to be applied to devices that are hooked up externally, typically though some form of computer bus like USB.
Typical examples include joysticks, printers and scanners. Devices such as monitorss and disk drives are not considered peripherals because they are not truly optional, and video capture cards are typically not referred to as peripheral because they are internal devices.