http://jakarta.apache.org/site/binindex.cgi wrote:
Release Builds are those that are ready for Prime Time. This build is "as good as it gets!"

Milestone Builds are those that are somewhat stable but not crystal-clean. We have some confidence in them, but they are buggy and should only be used by advanced users who want to explore future product direction or take advantage of new features. For developers, these builds serve as a mechanism to track progress towards a release.