Mirrors: standalone servers with complete copy of npm registry
Proxies: proxy to the database (couchdb) of npm registry, if only the npm registry server fails but the db works
See this gist: https://gist.github.com/3331671
(+) means server is self updating (pulls newest stuff from offical registry when back online again) -- Compared to mirrors with only snapshots that can contain older versions of modules
Proxies are/should be self-updating per definition.