WebDynpro - Dialog Boxes

Generally speaking , there are two different types of dialog boxes: Model and Modeless , WebDynpro support both of them.

Dialog boxes are implemented within a Web Dynpro application via an additional window and are generally called by the event handler of an action. The component controller contains the interface IF_WD_WINDOW_MANAGER, with which a new window for the content of the dialog box can be created and opened.

Three scenarioes:
1. Calling Dialog Boxes of the Same Component
2. Calling Dialog Boxes of a Used Component
3. Calling a Confirmation Dialog Box

Reference :
1. SAP Help