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
Getting Started
Visual Studio .NET
Events
Windows Forms
Dialog Boxes
Controls: The Base Class
Mouse Interaction
Text and Fonts
Drawing and GDI+
Labels and Buttons
Text Controls
Other Basic Controls
TreeView and ListView
List Controls
Date and Time Controls
Custom Controls
Menus and Bars
ADO.NET
Updating ADO.NET
Exceptions and Debugging
Configuration and Deployment