The YAHOO Global Object lives at the top of the namespace tree for all YUI components. It also includes some utility methods for type-checking, user-agent detection, and module extension.
YAHOO.langtype checking methods to deal with unpredictable data
YAHOO.env.uacan tell you with some accuracy which browser your page is being viewed in.
YAHOO.lang.extend: Leverage class inheritance in an object-oriented architecture.
YAHOO.lang.augmentPrototo help modularize class behaviors.
augmentObjectto extend static classes, object literals, or class instances.
YAHOO.lang.merge: Merge several objects, creating a unique set.
All YUI 2.x users should review the YUI 2.8.2 security bulletin, which discusses a vulnerability present in YUI 2.4.0-2.8.1.