Usually, WordPress theme developer already provide a tooltip for post title. This tutorial, a very easy tutorial, will help you add a fancy jQuery tooltip for your WordPress post title, replacing the default one. Read more: http://www.dynamicwp.net/articles-and-tutorials/how-to-create-fancy-jquerytooltip-for-your-wordpress-post-title/
Javascript
Put the code in the head tag
<script type="text/javascript">
function simple_tooltip(target_items, name){
jQuery(target_items).each(function(i){
jQuery("body").append("<div class='"+name+"' id='"+name+i+"'><p>"+jQuery(this).attr('title')+"</p></div>");
var my_tooltip = jQuery("#"+name+i);
if(jQuery(this).attr("title") != ""){ // checks if there is a title
jQuery(this).removeAttr("title").mouseover(function(){
my_tooltip.css({opacity:0.8, display:"none"}).fadeIn(340);
}).mousemove(function(kmouse){
my_tooltip.css({left:kmouse.pageX+15, top:kmouse.pageY+15});
}).mouseout(function(){
my_tooltip.fadeOut(200);
});
}
});
}
jQuery(document).ready(function(){
simple_tooltip(".entry-title a","title-tooltip");
});
</script>

Comments will be moderated and
rel="nofollow"will be added to all links. You can wrap your coding with[code][/code]to make use of built-in syntax highlighter.The easiest way is to use the pure CSS as follows:
<a href="http://www.menucool.com/tooltip/css-tooltip">CSS Tooltip</a>
Now CSS3 is available for most browsers. The following is also a good choice:
<a href="http://www.menucool.com/tooltip/css-tooltip">CSS Tooltip</a>
If you want a rich HTML tooltip that is cross browser compatible, use the following one:
<a href="http://www.menucool.com/tooltip/css-tooltip">CSS Tooltip</a>