What is a callback function?

Question

What is a callback function?

Answer 1

One important usage area is that you register one of your function as a handle (i.e. a callback) and then send a message / call some function to do some work or processing. Now after the processing is done, the called function would call our registered function (i.e. now call back is done), thus indicating us processing is done.
This wikipedia link explains quite well graphically.