What is CallBack - In General ?

Callbacks are used extensively in asynchronous programming. When you don't want to block until a (possibly) long-running operation completes, one of the ways to approach the problem is to delegate the operation to someone who will do it on the side for you. This raises the question: how will you be able to tell when the operation is complete, and how will you get its results?

One solution would be to delegate the work to someone else and take a moment off your normal work every now and then to ask "is the work I gave you done yet?". If so, get the results in some way and off you go. Problem solved.

The problem with this approach is that it doesn't make your life much easier. You are now forced to ask every little while and you will not know that the operation is done as soon as it actually is (but only the next time you remember to ask). If you forget to ask, you will never be notified.

A better solution to this is the callback: when delegating work, provide a function along with it. The code which will actually do the work then promises to call that function as soon as the work completes. You can now forget all about that stuff and be secure in the knowledge that when the work is done, your callback will be called. No sooner, and no later.