Ext JS is a JavaScript framework for building data-intensive cross-platform web applications for desktops, tablets, and smartphones. It provides a rich set of UI components and a comprehensive architecture for developing modern web applications with complex user interfaces and interactive data visualization. Ext JS's component-based approach enables developers to create reusable UI components and assemble them into sophisticated layouts with minimal effort. It offers features such as data binding, grid controls, charts, forms, trees, drag-and-drop functionality, and theming support. Ext JS applications can be packaged as standalone desktop applications using Electron or Cordova, allowing them to run outside the browser as native desktop or mobile apps. Ext JS is used by developers and organizations to build enterprise-grade web applications with rich user experiences and high performance.