A dialog box (also spelled dialogue box, also called a dialog) is a common type of window in the GUI (graphical user interface) of an operating system. The dialog box displays additional information, and asks a user for input.
For example, when you are using a program and you want to open a file, you interact with the "File Open" dialog box. In Microsoft Windows, when you right-click a file and choose Properties, you are presented with the Properties dialog box.
Modal and modeless dialogs
A modal dialog temporarily locks you out of the rest of an application until it's closed. While a modal dialog box is open, it's always active and prevents any other dialog boxes or windows from being opened until it is closed.
A modeless dialog lets you use the rest of the application while it is open. It can be hidden by other application windows.
A document-modal dialog is used in modern versions of macOS, and in some applications such as Opera. Instead of appearing in its own window, the dialog is presented as a "sheet" attached to the window. While it is open, a document-modal dialog locks the user out of the parent document window, but other document windows in the application can still be used. An example of a document-modal dialog is the "Save As" dialog in the macOS TextEdit application, shown below.