Dialogs are used to edit content and properties. Dialogs typically contain a form. Authors type content into the form fields and the dialog stores the content in the repository.
Magnolia provides standard dialogs for editing page properties, component content and many other editable items. You can also create your own dialogs. Instead of starting from scratch, extend an existing dialog definition and adapt it to your needs. You can find example dialogs in the Standard Templating Kit, Templating and Templating Samples modules.