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The effect consists of two kinds of elements - trigger elements and panel elements.
The triggers are links in the nav bar. Panels are usually a <div> with a unique ID - the big rectangles that have all the menu content in it.
When the page loads, style each panel with "display:none".
To each trigger, add a "mouseover" event that changes its corresponding panel to "display:block"
<a href="#" onmouseover="document.getElementById('panelname').style.display='block'"> roll over me </a> to each panel, add a "mouseout" event that adds change the panel back to "display:none".
<div id="panelname" style="display:none" onmouseout="this.style.display='none'"> link link link </div> That's a bare-bones example using inline styles and script. A more sophisticated attempt would put the CSS and JS into external files.