Windows CE includes several system-defined windows classes. Because Windows CE registers these classes, you do not need to; you can immediately create windows with them.
The simplest group of these built-in classes are the controls. Controls provide simple display and user interaction. You generally create controls as child windows of a more complicated window.
Dialog boxes are windows that manage a number of controls. You create them with a template that specifies which controls are to be included in the dialog box. The dialog box then manages the user interaction between the controls and the rest of the program.
Message boxes are highly specialized dialog boxes. They generally have minimal text and a few buttons. Because the system handles the details of creating and interacting with the dialog box, message boxes are easy to use.