Callback Functions

A callback function is code within a managed application that helps an unmanaged DLL function complete a task. Calls to a callback function pass indirectly from a managed application, through a DLL function, and back to the managed implementation. Some of the many DLL functions called with platform invoke require a callback function in managed code to run properly.
Generally speaking a callback is executable code that is passed as an argument to other code

See in the following function's signature:
              BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam)  
lp prefix comes from the long pointer term.Thus the lpEnumFunc is a pointer to a callback function.

The EnumWindows function enumerates through all existing windows on your computer, calling the callback function to perform a task on each window. Thiw function belongs to the Win32 API (user32.dll).
Also,in the following example:

EnumWindows  unmanaged DLL function. 

CallBack(int hwnd, int lparam) is the managed  callback function

hwnd:handle to the window

lparam:application defined parameter

using System;
using System.Runtime.InteropServices;

public delegate bool CallBack(int hwnd, int lParam);

public class EnumReportApp
{
    [DllImport("user32")]
    public static extern int EnumWindows(CallBack x, int y); 

    public static void Main() 
    {
        CallBack myCallBack = new CallBack(EnumReportApp.Report);
        EnumWindows(myCallBack, 0);
    }

    public static bool Report(int hwnd, int lParam)
    { 
        Console.Write("Window handle is ");
        Console.WriteLine(hwnd);

        //Callback functions generally return nonzero values to indicate success         //and zero to indicate failure. This example explicitly sets the return v        //alue to true to continue the enumeration.

        return true;
    }
}

Similar enumeration functions are: EnumFontFamilies, EnumPrinters

The usage of a callback is usually in asynchronous logic.