A peripheral is a device that is connected to a host computer, but not part of it. It expands the host's capabilities but does not form part of the core computer architecture. It is often, but not always, partially or completely dependent on the host.
There are three different types of peripherals:
- Input, used to interact with, or send data to the computer (mouse, keyboards, etc.)
- Output, which provides output to the user from the computer (monitors, printers, etc.)
- Storage, which stores data processed by the computer (hard drives, flash drives, etc.)
A peripheral is generally defined as any auxiliary device such as, for example, a computer mouse or keyboard, that connects to and works with the computer in some way. Other examples of peripherals are expansion cards, graphics cards, computers, image scanners, tape drives, microphones, loudspeakers, webcams, and digital cameras. RAM - random access memory - straddles the line between peripheral and primary component; it is technically a storage peripheral, but is required for every major function of a modern computer and removing the RAM will effectively disable any modern machine. Many new devices such as smartphones and tablet computers have interfaces which allow them to be used as a peripheral by a full computer, though they are not host-dependent as other peripheral devices are. According to the most technical definition, the only pieces of a computer not considered to be peripherals are the central processing unit, power supply, motherboard, and computer case.
Usually, the word peripheral is used to refer to a device external to the computer case, like a scanner, but the devices located inside the computer case are also technically peripherals. Devices that exist outside the computer case are called external peripherals, or auxiliary components. Examples: "Many of the external peripherals I own, such as my scanner and printer, connect to the peripheral ports on the back of my computer." Devices that are inside the case such as internal hard drives or CD-ROM drives are also peripherals in technical terms and are called internal peripherals, but may not be recognized as peripherals by laypeople.
In a system on a chip, peripherals are incorporated into the same integrated circuit as the central processing unit. They are still referred to as "peripherals" despite being permanently attached to (and in some sense part of) their host processor.
Common peripherals 
Peripheral devices connect to the system unit through the ports on the computer.
See also 
- "Peripheral Definition". Pcsupport.about.com. 2012-04-10. Retrieved 2012-11-02.
- Technology In Action, Tenth edition.
- The dictionary definition of peripheral at Wiktionary
- List of Input Devices, Output Devices and Both Input Output devices related to computer.