A tooltip is a short and informative text that appears in a box that pops out when you hover the mouse cursor over an element (link, button, div etc.).
1. DHTML JavaScript Tool Tips is a JavaScript Cross Browser library to create tool tips (information pop-up boxes). This script can be customized in multiple ways and may contain plain-text as well as HTML (images, line breaks).

* Implementation
1. Download the JS File: wz_tooltip.js.
2. Include the library in your HTML page by inserting the following line in the BODY section of your page (it can be added after the last tag containing a tool tip):
<script language="JavaScript" type="text/javascript" src="http://www.your-domain-name.com/path/to/wz_tooltip.js"></script>
3. Each of the tags requires only the onMouseOver attribute (onMouseOut is not required):
<a href="photos.htm" onmouseover="return escape('Your text here')">Photos</a>
* Browsers Compatibility: Netscape 4 and 6+, Mozilla, Galeon, Opera 5, 6 and 7+, MSIE 4, 5+, Konqueror 3
2. Fading JavaScript Tooltips 2kb is an animated JS tool tip script that is very easy to use and lightweight. To see how the script is implemented check the index.html file from the ZIP archive.

Tested in: IE6+, FireFox, Opera, Safari
To see a walk-through of the code and the logic check this post.
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