Muffins are like cookies, but larger. ;D
They’re for non-signed (ie sandboxed) webstart apps to provide them with a small amount of local storage since they can’t access the local file system. I think Kev had some muffin code knocking around the shared code forum a while ago.
http://java.sun.com/j2se/1.5.0/docs/guide/javaws/developersguide/examples.html#PersistenceService