Messages are a fundamental part of the operating system. PM applications use messages to communicate with the operating system and one another. The system uses messages to communicate with applications to ensure concurrent running and sharing of devices. Typically, a message notifies the receiving application that an event has occurred. The operating system identifies the appropriate application window to receive a message by the window handle included in the message. Sources of events that cause messages to be issued to applications are the user, the operating system, the application, or another application.