A peripheral is a piece of computer hardware that is added to a host computer, i.e any hardware except 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 through some form of computer bus loke USB. Typical examples include joysticks, printers and scanners. Devices such as monitors and disk drives are not considered peripherals when they are not truly optional.
Some people do not consider internal devices such as video capture cards to be peripherals because they are added inside the computer case; for them, the term peripherals is reserved exclusively for devices that are hooked up externally to the computer. It is debatable however whether PCMIA cards qualify as peripherals under this restrictive definition, because some of them go fully inside the laptop, while some, like WiFi cards, have external appendages.
Due to these considerations, the term has somewhat fallen out of modern usage; it was mainly used in the early days of the home computer boom to refer to things like floppy disk drives and modems. Today most devices are integrated inside the computer's case, and external devices typically function on their own such as mp3 players and cell phones.
The term is different from computer accessories: Computer peripheral has a narrow meaning that refers only to the input output devices of a computer, whereas, computer accessories has a broader meaning, that refers, all the parts that support a computer which includes motherboards, sensors, chips, including all the input and output devices. See http://en.wikipedia.org/wiki/Peripheral
By Alan Ball, ON Semiconductor Computer boards and peripherals interface with the internal power sources of a computer. There is a need for local protection circuitry whenever a device is interfaced to a power source with multiple loads.