Ext JS is a JavaScript library for building richly interactive web applications using techniques such as AJAX, DHTML and DOM scripting.
Originally built as an extension of YUI, Ext can now also extend jQuery and Prototype. As of version 1.1, Ext can run stand-alone without relying on any of those external libraries, though they remain an option for integration.
Ext version 2.0 can now use many different base libraries or adapters) (e.g. YUI, jQuery, Prototype), or it can work standalone.