Ext JS (Ext) is a javascript-based framework for developing Rich Internet Applications, and it’s simply wonderful.
Other Javascript frameworks like JQuery, Prototype and many others are powerful and let you make web pages cooler and more interactive, but their limits are evident. Yes, your web pages will be shorter than plain javascript files but if you wanna develop web applications or someother more complex, you need other, you need Ext.
Ext has many built-in components, which enables you with only a little lines of javascript to create an evoluted graphical user interface, with associated events. You can build forms with embedded form validation, data grid with dynamic options like editing or sorting, or you can simulate a desktop environment with windows in a web page. And more: feed readers, tabbed windows, trees, dynamic loaded combos, and all you can need. It’s also skinnable and customizable. It’s cross-browser compatible, and you don’t need to worry about browser incompatibilities.
And the magic of this tool is that you can forget all javascript functions, you’ll need to know only javascript and json syntax, all the rest is Ext and only Ext.
It’s used by many important and famous customers and that’s a sign of reliability and powerfulness of this framework. You find the customers list in home page. There’ s a complete and well explained documentation, and there are also nice examples to begin learning, some tutorials and a forum to get support.

A comparison between some frameworks
Read a related discussion about ajax frameworks (Italian)
Another review (Italian)

I’m an ExtJS developer, so contact me to get consultancy.