In Java, a package is a mechanism to encapsulate a group of related classes, sub-packages, and interfaces. It is used to group related classes together, similar to how folders organize files in a file directory. Packages provide a unique namespace for each type they contain, and classes in the same package can access each others package-private and protected members.

Packages can contain classes, interfaces, enumerations, and annotation types. They allow developers to group classes together that are related in some way, such as performing a specific set of tasks or belonging to the same category. Packages also help to avoid naming conflicts and write maintainable code.

Java has two types of packages: built-in packages and user-defined packages. Built-in packages are part of the Java API and contain prewritten classes that are free to use. User-defined packages are created by developers and stored in a file system directory. To create a package, the package keyword is used.

Using packages is considered good practice in Java programming, as it helps to organize code and make it more manageable. It also allows for better access protection and removes naming collisions.