*Basic

I/O devices are the pieces of hardware used by a human to communicate with a computer. For instance, a keyboard and a mouse is an input device for a computer, while monitors and printers are output devices.[1]

**Intermediate

Devices for communication between computers, such as modem and network cards, typically perform both input and output operations.[2] Mice and keyboards take physical movements that the user outputs and convert them into input signals that a computer can understand, the output from these devices is the computer's input.[3] Input/output devices are directly connected to an electronic module inside the systems unit called a device controller. For example, the speakers of a multimedia computer system are directly connected to a device controller called an audio card, which in turn is connected to the rest of the system.[4]

***Advanced

Sometimes secondary memory devices like the hard disk are called I/O devices because they move data in and out of main memory.[5] What counts as an I/O device depends on context. To a user, an I/O device is something outside the system box. To a programmer, everything outside the processor and the main memory looks like an I/O device. To an engineer working on the design of the processor, everything outide of the processor is an I/O device.[6]

Sources

[1] Editors. Input/Output. Wikipedia. en.wikipedia.org
[2] Editors (2017) Input/Output Device. Technopedia. Technopedia.com
[3] Editors. Operating System – I/O Hardware. Tutorials Point. Tutorialspoint.com
[4] Editors. Input/Output Controllers. Chortle. Chortle.ccsu.edu
[5] Willmann, P et al. Protection strategies for direct access to virtualized I/O devices. Rice University
[6] Editors. I/O devices. IBM. Ibm.com