The computer sits between the input and output devices. A computer program processes and digitizes the input information. The result is then sent to an output device such as a screen.
Think of a DVD player
You click ‘play’ on your remote control, an input device. It digitizes the input and sends this digital information to the DVD player. The computer inside the DVD player processes this input information and works out what to do. It will start to read the DVD. The video and audio information from the DVD is then sent to the TV screen and the speakers, the output devices.
Think of a games console:
When you are playing a game you are using a controller where you push a button or push the control stick to move a character. These inputs are digitized and sent to the computer to be processed. The program processes the inputs and gives an output. In this case the character on screen moves as it has been programmed to. An input device is something you connect to a computer that sends information into the computer. An output device is something you connect to a computer that has information sent to it.