Window Messages

The operating system communicates with your application window by passing messages to it. A message is simply a numeric code that designates a particular event. For example, if the user presses the left mouse button, the window receives a message with the following message code.

#define WM_LBUTTONDOWN    0x0201

Some messages have data associated with them. For example, the WM_LBUTTONDOWN message includes the x-coordinate and y-coordinate of the mouse cursor. 

To pass a message to a window, the operating system calls the window procedure registered for that window.