This document discusses using the Yahoo! User Interface (YUI) library to build rich interactive web applications. It covers using YUI to send and receive data via AJAX, build interfaces with components like tabs and dialogs, display data visually with charts and calendars, and modify data by connecting forms to backends. Examples show integrating YUI widgets like the DataTable and Calendar into pages to enhance functionality and interactivity. Resources listed at the end provide documentation, examples and support for further YUI development.