EMC2 is now using a "major/minor" release system.
EMC version numbers work like this:
Minor releases are for bugfixes only, not for new features. Our criteria for minor releases is that users should not have to change their ini files or other configuration data to move from one minor release to the next. If 2.1.5 worked for you, you can use 2.1.6 or 2.1.7 without changing anything. You should always use the latest minor release (and if you are using Ubuntu you will get automatic updates to the latest minor release).
Major releases add new features, and may not be 100% compatible with the previous major release. When there is incompatibility we describe the necessary configuration changes in the wiki.
Changes to go from prior versions to 2.3: http://wiki.linuxcnc.org/cgi-bin/wiki.pl?UPDATING
Changes to go from prior versions to the current development version: http://wiki.linuxcnc.org/cgi-bin/wiki.pl?UpdatingConfigurationsForDevelopmentVersions
We try to avoid incompatibility between major releases as much as possible, but sometimes new features mean that something MUST change. You should update to a new major release only after considering the benefits of the new features versus any configuration changes that are needed. You will NOT be automatically updated from one major release to another - we leave that decision to you.
Most users should be using a Released version of EMC2. Released versions are tested by both developers and beta users before being released, and are suitable for the average user. Most developers and IRC/mailing list regulars are able to help support people running a released version. Released versions are available in several forms, including .debs for ubuntu, and source tarballs for local compilation. There will be a debian repository which will always have the latest released version (and thus allows for easy upgrades from one stable release to the next).
You can also run the latest unreleased version of EMC. Unreleased versions come directly from the Git server, and have a version number that contains a tilde, such as "2.4~git". Unreleased version are the latest code, with new features (and probably new bugs), and are not recommended for production machines. Use an unreleased version only if you need the new features or want to help test out the latest code before it is released.