Here is one solution using the qtip [tooltip] plugin. See http://craigsworks.com/projects/qtip/docs/ for docs.
CSS:
ul { margin: 20px; }
li { padding: 5px; float: left; border-left: 1px dotted #d0d0d0; background-color: green; }
li:hover { background-color: yellow; }
li a { color: white; text-decoration: none; }
li a:hover { color: black; }
#excerpt { display: none; }
HTML:
<ul id="menu">
<li><a rel=".excerpt-1" href="#">Nav Item 1</a></li>
<li><a rel=".excerpt-2" href="#">Nav Item 2</a></li>
<li><a rel=".excerpt-3" href="#">Nav Item 3</a></li>
</ul>
<div id="excerpt">
<div class="excerpt-1">EXCERPT 1 - Lorem ipsum dolor sit amet</div>
<div class="excerpt-2">EXCERPT 2 - Lorem ipsum dolor sit amet</div>
<div class="excerpt-3">EXCERPT 3 - Lorem ipsum dolor sit amet</div>
</div>
--
Render your content inside #excerpt.
--
JS:
$(document).ready(function() {
$('#menu li a').each(function() {
var id = $(this).attr('rel');
$(this).qtip({
content: $(id).html(),
style: { name: 'dark' }
});
});
$('#menu li a').click(function() { return false; });
});
--
Hope this helps. Working demo is here: http://jsfiddle.net/3vrjL/