Yup. Whereas the normal calling sequence for an API is "Don't call us, we'll call you", using callbacks is more "If something interesting happens, call me at this number (coderef)". One good way to get your head into the callback-driven mindset is to use one of the GUI toolkits which are pretty much all event driven. When something interesting happens (the user hits the mouse button; the environment needs part of a window redrawn) the framework needs a chunk of code (the callback) which it calls into in order to handle the event.