Updates via email: subscribe to this mailing list be notified when existing content changes. The idea is to complement the feeds where you get no such notifications (in contrast to Twitter and Google+).
@2ality (Twitter): tells you when new content is published or existing content is changed.
Google+: has the same content as @2ality on Twitter.
About feeds (RSS, Atom):
Feeds allows you to receive news in a fashion similar to email (unread messages etc.).