There are a few final pieces that of the object-oriented puzzle we need, in order to keep a solid grasp of our goals. We've talked about how Ext JS is a large collection of objects extending other objects, but it's also important to understand a few other pieces of basic OO terminology, and how they help in keeping things organized.
A package is a collection of classes that share something in common. For instance, the Ext.data package is a collection of classes for dealing with data, such as the different types of data Stores, Readers, and Records. The Ext.grid package is a collection of classes for the various grid objects, including all of the different grid types and selection models. Likewise, the Ext.form package contains classes for building forms, to include all of the classes of the different field types.