What is "extjs-4"

Sencha Extjs provides a complete object-oriented structure for the creation of a desktop application , as it runs on a web browser or packaged as a native application. It manages the object life cycle, layouts , thematization , data storage, ajax communication and a large library of user interface widgets, including graphics.

Originally built as an add-on for YUI library, it has a modular architecture that developers can extend using Javascript.

Source: extjs