You know that PubSubHubbub has something to do with RSS feeds but what exactly is PubSubHubbub and how does it help the real-time web? Watch the video.
Blogger, Google Reader, Google Alerts, FriendFeed and FeedBurner are some products that are PubSubHubbub enabled. This basically means that as soon as you post something, it will immediately appear in the feed reader that supports the PubSubHubbub protocol.
A subscriber initially pulls the Atom or RSS feed in the conventional way, i.e. by requesting it from the feed server. The subscriber then inspects the feed, and if it references a hub, the subscriber can subscribe to the feed URL topic on that hub. The subscriber runs a server so that hubs can directly notify it when any of its subscribed topics have updated.
Publishers expose their content as Atom or RSS feeds, but with the inclusion of hub references. They post notifications to those referenced hubs whenever they publish something. Thus, when a publication event occurs, the publisher calls its hubs and the hubs call their subscribers.
Here’s another presentation deck that explains Pubsubhubbub.