A simple, open, server-to-server web-hook-based pubsub (publish/subscribe) protocol as an extension to Atom and RSS.
"There are three parties in the Pubsubhubbub model. There's a Publisher (FeedBurner, for example), and a Subscriber (perhaps Netvibes) and the communication is facilitated through a Hub (Google's AppSpot Hub is the demo and most popular Hub so far). The publisher knows that every time new content is published it's going to notify the Hub - the Hub that gets notified will be declared at the top of the publisher's document, just like an RSS feed URL. So the Publisher delivers new content to the Hub and then the Hub will deliver that message immediately to all the Subscribers who have subscribed to recieve updates from that particular publisher. This is very different from the traditional model of a subscriber polling a publisher directly every 5 to 30 minutes (or less) to check and see if there's new content. There usually isn't and so that model is inefficient and slow. Hubbub is nearly immediate and only takes action when something important occurrs. It's remarkably simple, at the end points in particular. If things ever get complicated it's in the Hub and that's easily available as a service if a publisher doesn't want to host their own".
Source: Explanation of pubsubhubub.