An output device is simply something that gives out an information flow of some kind, usually of information processed by another object or system. An example would be headphones: An mp3 player provides a stream of information, but the headphones are the output device that allows us to hear it.
The output device means the feed back of the computer of our questions(reactions). We can take example of the any short program of the C++ language, if we are running a program of the addition then what input we gave for the addition form the keyboard suddenly the answer displays on your screen. In this way your monitor is input as well as output device. Printer,teller machine etc, are the output device.