The processing function then calls the callback when appropriate. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. Other toolkits achieve this kind of communication using callbacks.
For example, if a user clicks a Close button, we probably want the window's close() function to be called. More generally, we want objects of any kind to be able to communicate with one another.
In GUI programming, when we change one widget, we often want another widget to be notified.
