An I/O device, or Input/Output device, is a hardware component that allows a computer to communicate with the outside world. Input devices, like keyboards and mice, enable users to send data to the computer, while output devices, such as monitors and printers, display or produce results from the computer's processing. These devices are essential for user interaction and data exchange. They help in executing commands, displaying information, and facilitating communication between the computer and its users. Without I/O devices, computers would be unable to receive input or provide output, limiting their functionality.