From the Wikipedia article on Java package naming:
In general, a package name begins with the top level domain name of the organization and then the organization's domain and then any subdomains, listed in reverse order. The organization can then choose a specific name for its package. Package names should be all lowercase characters whenever possible.
For example, if an organization in Canada called MySoft creates a package to deal with fractions, naming the package ca.mysoft.fractions distinguishes the fractions package from another similar package created by another company. If a US company named MySoft also creates a fractions package, but names it us.mysoft.fractions, then the classes in these two packages are defined in a unique and separate namespace.