Question

what is JQUERY???? how to install it

Answer 1

It's a javascript library. You can download it here:
http://docs.jquery.com/Downloading_jQuery#Download_jQuery
Or you can link it in the head section of your webpage with the following line:
<script type='text/javascript' src=' http://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js' ></script>

There is no installation, jou just put it in some folder acessible via your webserver (on linux it's usually /var/www/, on windows xampp installation, it's C:\xampp\htdocs, or something similar, and on paid hosting, it's usually <your home folder>/public_html)