Let's start with the definition of a "Java package", as described in the Wikipedia article:
A Java package is a mechanism for
organizing Java classes into
namespaces similar to the modules of
Modula. Java packages can be stored in
compressed files called JAR files,
allowing classes to download faster as
a group rather than one at a time.
Programmers also typically use
packages to organize classes belonging
to the same category or providing
similar functionality.
So based on that, packages in Java are simply a mechanism used to organize classes and prevent class name collisions. You can name them anything you wish, but Sun has published some naming conventions that you should use when naming packages:
Packages
The prefix of a unique package name is
always written in all-lowercase ASCII
letters and should be one of the
top-level domain names, currently com,
edu, gov, mil, net, org, or one of the
English two-letter codes identifying
countries as specified in ISO Standard
3166, 1981.
Subsequent components of the package
name vary according to an
organization's own internal naming
conventions. Such conventions might
specify that certain directory name
components be division, department,
project, machine, or login names.
Examples:
com.sun.eng
com.apple.quicktime.v2
edu.cmu.cs.bovik.cheese
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