The ImageJ Updater is a mechanism to update individual packages in ImageJ. The Updater keeps users up-to-date with all components of ImageJ, including both plugins and the core components (libraries) needed by the plugins.
The ImageJ Updater can handle 3rd-party update sites: anybody with write access to a web server can set up their own update site which users can decide to follow.
For more details, see the Updater page on the ImageJ wiki.