Extending jQuery with UI elements
So we all know jQuery, and the magic it can do with javascript and DOM manipulation. If you want to use some custom UI elements, you usually have to go find a proper plugin, and hope its supported, and that it still lives(if open source project). Why not just use the ones that are supported by jQuery, and have their full support. jQuery UI makes it very simple to implement some plugins, that are often used. i.e. a datepicker, a dialog(alert done nice), or a progressbar.

jQuery UI also lives in several CDN's, making it very easy to implement on your site, it can be done like this

     <link rel="stylesheet" type="text/css" href="//ajax.aspnetcdn.com/ajax/jquery.ui/1.8.17/themes/smoothness/jquery-ui.css" /> 
     <script type="text/javascript" src="//ajax.aspnetcdn.com/ajax/jQuery/jquery-1.7.1.js"></script>
     <script type="text/javascript" src="//ajax.aspnetcdn.com/ajax/jquery.ui/1.8.17/jquery-ui.min.js"></script>

The UI also offers different themes for the controls, and offers localization aswell.

Check it out at http://jqueryui.com/