Briefly explained: “Input and output units enable the exchange of information between the computer and its environment.”

The task of INPUT devices is to transmit data to the computer in a form it can understand.

The task of the OUTPUT units is to present the output data in a form understandable to the user or to store it.

Input/Output devices perform both of the above tasks!

Source (Slovenian language): http://colos.fri.uni-lj.si/eri/RACUNALNISTVO/ZGRADBA_RACUNALNIKA/vhodIzhod.html