The document discusses how to create custom modules for YUI3. It covers adding a module using YUI.add(), namespacing, adding version and dependency information. It also discusses augmenting modules using the YUI Base class, which provides attributes, events, and initialization methods. The document provides examples of different loading strategies and introduces the YUI Gallery for sharing modules between developers by including them in a YUI.use statement. It outlines the benefits of contributing to the gallery and the steps to do so, including creating an account, forking YUI3 on GitHub, submitting the module, and having it approved to be hosted on Yahoo's CDN.