Windows uses two methods to route messages to a window procedure: posting messages to a first-in, first-out queue called a message queue, a system-defined memory object that temporarily stores messages, and sending messages directly to a window procedure.
Messages posted to a message queue are called queued messages. They are primarily the result of user input entered through the mouse or keyboard, such as WM_MOUSEMOVE, WM_LBUTTONDOWN, WM_KEYDOWN, and WM_CHAR messages. Other queued messages include the timer, paint, and quit messages: WM_TIMER, WM_PAINT, and WM_QUIT. Most other messages, which are sent directly to a window procedure, are called nonqueued messages.