According to MSDN

The LowLevelMouseProc hook procedure is an application-defined or library-defined callback
function used with the SetWindowsHookEx function. The system call this function every time
a new mouse input event is about to be posted into a thread input queue. The mouse input
can come from the local mouse driver or from calls to the mouse_event function. If the input
comes from a call to mouse_event, the input was "injected". However, the WH_MOUSE_LL
hook is not injected into another process. Instead, the context switches back to the
process that installed the hook and it is called in its original context. Then the context
switches back to the application that generated the event.


The HOOKPROC type defines a pointer to this callback function. LowLevelMouseProc is a
placeholder for the application-defined or library-defined function name.


Code:
LRESULT CALLBACK LowLevelMouseProc(          int nCode,
    WPARAM wParam,
    LPARAM lParam
);

Mouse Proc

The MouseProc hook procedure is an application-defined or library-defined callback function
used with the SetWindowsHookEx function. The system calls this function whenever an
application calls the GetMessage or PeekMessage function and there is a mouse message
to be processed.

The HOOKPROC type defines a pointer to this callback function. MouseProc is a placeholder
for the application-defined or library-defined function name.


Code:
LRESULT CALLBACK MouseProc(          int nCode,
    WPARAM wParam,
    LPARAM lParam
);