A null link is an undesignated link. Use null links to attach behaviors to objects or text on a page. For instance, you can attach a behavior to a null link so that it swaps an image or displays an absolutely-positioned element (AP element) when the pointer moves over the link.
Script links execute JavaScript code or call a JavaScript function and are useful for giving visitors additional information about an item without leaving the current web page. Script links can also be used to perform calculations, validate forms, or do other processing tasks when a visitor clicks a specific item.