A dialog box is a window that appears on top of a main application or other window (or occasionally no window at all) requesting user input. Dialog boxes are characterized by one or more input fields, such as edit boxes, check boxes, selection lists and others, an “OK” or equivalent button, and a “Cancel” button. Dialog boxes may or may not have title bars across the top, and while usually movable, they are most often not resizable.
If presented by an application, a dialog box is typically smaller than the application window, so as to appear to be a part of that application.
The term “dialog” refers to the metaphor that the computer is having a conversation or dialog with the user, and is asking for some form of input and processing that input when supplied.