In a graphical user interface of computers, a dialog box is a type of window used to enable reciprocal communication or "dialog" between a computer and its user. It may communicate information to the user, prompt the user for a response, or both. A dialog box is most often used to provide the user with the means for specifying how to implement a command, or to respond to a question or an "alert".
Questions having the dialog tag regards the User Experience of the UI-element dialog (box).
Reference: Wikipedia: Dialog box