After some optional processing, it will call DispatchMessage(), which dispatches the message to the relevant handler, also known as WindowProc. Normally, messages that have no special WindowProc() are dispatched to DefWindowProc, the default one.
DispatchMessage() is invoked after some preliminary processing, which then forwards the message to the relevant handler, also denoted as WindowProc. Usually, messages lacking a specific WindowProc() are routed to DefWindowProc, the default method.