First, let's cover the Atom packages. If you need to build a Java application that acts as an Atom protocol client, then use the Atom client package {@link com.sun.syndication.propono.atom.client}. As you can see from the dependency diagram below, it depends on the Atom common package {@link com.sun.syndication.propono.atom.common} and it uses ROME, JDOM and Apache HTTP Client.
If you are adding Atom server capabilities to your Java web application, then you'll need to use the Atom server package {@link com.sun.syndication.propono.atom.server}. It depenends on ROME and JDOM.
We provide a file-based Atom server impementation in {@link com.sun.syndication.propono.atom.server.impl}, which implements the Atom interfaces defined in the server package and provides a simple Atom server that stores entries and associated media files on disk.
Now, let's cover the Blog Client packages. The main package is {@link com.sun.syndication.propono.blogclient}. You'll only need to use to the Blog Client package if you are publishing to legacy systems that don't yet support Atom protocol. By using the Blog Client you can write code that works against both old blog servers that support MetaWeblog API and never servers that support Atom protocol.
As you can see from the dependency diagram below, the MetaWeblog API implementation of the Blog Client uses Apache XMLRPC 3.0 and the Atom protocol implementation uses the Atom client.
For more information on the packages, refer to the Javadoc package summary pages.