This document discusses packages in Java. It defines a package as a mechanism to encapsulate a group of classes, subpackages, and interfaces. Packages are used to prevent naming conflicts, make classes and interfaces easier to locate and use, and provide access control. Package names are related to directory structure, and packages can be considered a form of data encapsulation. The document also discusses built-in packages that are part of the Java API, as well as how to define and use user-defined packages through directory structure and import statements.