In computer programming, a callback is executable code that is passed as an argument to other code. It allows a lower-level software layer to call a subroutine (or function) defined in a higher-level layer.
Usually, the higher-level code starts by calling a function within the lower-level code passing to it a pointer or handle to another function. While the lower-level function executes, it may call the passed-in function any number of times to perform some subtask. In another scenario, the lower-level function registers the passed-in function as a handler that is to be called asynchronously by the lower-level at a later time in reaction to something.
A callback can be used as a simpler alternative to polymorphism and generic programming, in that the exact behavior of a function can be dynamically determined by passing different (yet compatible) function pointers or handles to the lower-level function. This can be a very powerful technique for code reuse.
Reference:
http://en.wikipedia.org/wiki/Callback_%28computer_science%29
