The behaviors and uses of alerts differ from those of dialog boxes. Important distinctions also exist between different types of alerts and between different types of dialog boxes. You choose among these according to the user's current situation. Your application should give an alert to report an error or to issue a warning to the user.
Alert boxes are special windows that contain informative text, buttons, and, generally, icons.
They may also contain pictures. An alert box used by the Finder By requiring the user to click a button, an alert box obliges the user to acknowledge the alert box before proceeding. The Dialog Manager draws a bold outline around the preferred button so that it stands out from the other buttons in the alert box. Use a dialog box when your application needs more information to carry out a command. Commands in menus normally act on only one object.