How can I miss you when you won't go away?
Dan Hicks, "Original Recordings"
A dialog box is a simple form, though some dialog boxes are simpler than others. Typically, a dialog box either displays a message or gathers a bit of information. In either case, the dialog box "pops up" in the middle of your application to serve a specific targeted purpose.
A dialog box is distinguished from a form primarily by appearance (it is typically smaller and simpler than a full-form window) and function (it typically does only one thing), but the truth is there is no hard and fast distinction between a dialog box and a form. Both derive from the Form class.
That said, most dialog boxes have the following traits. They:
Windows Forms and the .NET Framework
Visual Studio .NET
Controls: The Base Class
Text and Fonts
Drawing and GDI+
Labels and Buttons
Other Basic Controls
TreeView and ListView
Date and Time Controls
Menus and Bars
Exceptions and Debugging
Configuration and Deployment