This document discusses developing components and extensions for Ext JS. It begins with introductions and an example of how the presenter first encountered and used Ext JS for an internal portal application. It then covers Ext JS terminology like components, containers, extensions, plugins and mixins. It provides examples of creating a simple clock component extension with steps like identifying a base class, creating class files, and adding interactivity with plugins. The document emphasizes that extensions allow reuse and customization for other developers, and provides advanced extension examples like Ext Calendar Pro and Ext Scheduler.