The purpose of an alert is to notify the user whenever an unusual situation occurs.
An alert box looks like a modal dialog box, except that it's somewhat narrower and appears lower on the screen. An alert box is primarily a one way communication from the system to the user; the only way the user can respond is by clicking buttons. Therefore alert boxes might contain dials and buttons, but usually no editable text fields, radio buttons, or check boxes.
How the buttons in an alert box are labeled depends on the nature of the box. If the box presents the user with a situation in which no alternative actions are available, the box has a single button that's labeled OK. Clicking this button means "I've read the alert." If the user is given alternatives, then typically the alert is phrased as a question that can be answered "yes" or "no."
The preferred (safest) button to use in the current situation is boldly outlined. This is the alert's default button; its effect occurs if the user presses Return or Enter.
