The ModalPopup extender allows you to display content in an element that mimics a modal dialog box, which prevents a user from interacting with the rest of pages. The modal content can contain any hierarchy of controls. It is displayed above background (in z-order) that can have a custom style applied to it. Clicking OK or Cancel in the modal popup dismisses the content and optionally runs a custom script. The custom script is typically used to apply changes that were made in the modal popup. If a postback is required, you can allow the OK or Cancel control to perform a postback. By default, the modal content is centered on the page. However, you can set absolute positiniong and set only X or Y to center the content vertically or horizontally.