Javascript Pop up Boxes

Till now we have used only one kind of output statement - document.write. Javascript has few more "interactive" output statements in the form of Pop up Boxes. There are three types of Pop up Boxes - Alert box, Confirm Box and Prompt Box. Let us take a look at the Alert Box.A javascript alert box pops-up with a message and an 'OK' button. It displays an alert box with a string passed to it. For example: alert() will display an empty dialog box with just the 'OK' button. The alert("Hello world") will display a dialog box with the message, 'Hello world' and an 'OK' button.
http://www.referencedesigner.com/tutorials/js/js_8.php