Peripheral units or devices are the hardware components used for input or output of data. These are keyboard, disk drives, printers and plotters. They are connected to the main memory. They perform three main functions one is input, output and backing storage. Input devices provide the means whereby the human user can communicate with the computer. The output devices provide the means for the computer to communicate with the human users. The backing storage devices are external memory units which functions as extensions to the primary memory.

Backing storage devices are necessary because the capacity of the random access memory is limited and only those programs are loaded into random access memories which are currently being used and executed. Random access memory is volatile and its contents are lost when power supply is cut off. With these constraints of random access memory there is no way that all the data needed could be held in a computer at the same time. So it is necessary that any program or data that needs to be preserved must be copied on to a non-volatile from the storage. These storage devices are also called auxiliary storage devices. Apart from these, there are also other forms of input and output devices used with the computer.

 Peripheral Devices: In computer's hardware language the peripheral devices can be defined as the devices that are connected to the computer in order to get most of the advantage out of it,if you in to detail description of peripheral devices it can be explained as the devices that are optional, and which are not required in principle. In earlier ages when computers were too much expensive and personal computer were very hard to afford, at that time the Motherboard, CPU (Central Processing Unit) and Memory (RAM (random access memory) & ROM (read only memory)) were considered to be the main components of the computer and any other device additionally attached to the computer were considered as peripheral device so this means that the keyboard, mice etc at that time were considered as peripheral devices but now a days generally they are not considered peripheral devices.
The common peripheral devices are microphones, cameras, disk drives, scanners etc.
There is a common understanding that among people that the internal devices such as Sound Cards etc are not peripheral because they are added in the computer case which is wrong following is the peripheral devices list for a general idea.

(1) CD-ROM
(2) CD-R
(3) DVD-ROM
(4) USB flash drive
(5) Graphics Cards
(6) Sound Cards
(7) Modem
(8) Network Card
(9) Doking Station
Are some of the examples of peripheral devices.