This tells the operating system about your window

    // use 'LoadImage' to load wnd class icon and cursor as it supercedes 
    // the obsolete functions 'LoadIcon' and 'LoadCursor', although these 
    // functions will still work. Because the icon and cursor are loaded 
    // from system resources ie they are shared, it is not necessary to 
    // free the image resources with either 'DestroyIcon' or 'DestroyCursor'.


    hIcon=(HICON)LoadImage(0,IDI_APPLICATION,IMAGE_ICON,0,0,LR_SHARED);
    hCursor=(HCURSOR)LoadImage(0,IDC_ARROW,IMAGE_CURSOR,0,0,LR_SHARED);
    wcx.cbSize           = sizeof(WNDCLASSEX);          //byte size of WNDCLASSEX struct
    wcx.style            = CS_HREDRAW|CS_VREDRAW;       //ensure wnd is always redrawn
    wcx.lpfnWndProc      = (WNDPROC)MainWndWndProc;     //pointer to the Window Procedure
    wcx.cbClsExtra       = 0;                           //Extra bytes for 'class' wnds
    wcx.cbWndExtra       = 0;                           //Extra bytes for this wnd
    wcx.hInstance        = hInstance;                   //Application instance
    wcx.hIcon            = hIcon;                       //Application icon
    wcx.hCursor          = hCursor;                     //Cursor for wnd
    wcx.hbrBackground    = (HBRUSH)(COLOR_BTNFACE+1);   //Background wnd colour
    wcx.lpszMenuName     = TEXT("MainMenu");            //Name of window menu
    wcx.lpszClassName    = chClassName;                 //Name of this wnd 'class'
    wcx.hIconSm          = NULL;                        //Icon in top-left corner of wnd


    //Register the wnd class with the Windows system
    if (!RegisterClassEx(&wcx)) {
        //Registration has failed so inform the user
        MessageBox( NULL,
                    TEXT("Failed to register wnd class"),
                    TEXT("ERROR"),
                    MB_OK|MB_ICONERROR);
        return FALSE;
    }