Pubsubhubbub is an open, simple web-scale pubsub protocol. A common use case to enable blogs (Publishers) to "push" updates from their RSS or Atom feeds (Topics) to end Subscribers. These Subscribers will have subscribed to the blog's RSS or Atom feed via a Hub, a central server which is notified of any updates by the Publisher and which then distributes these updates to all Subscribers. Any feed may advertise that it supports one or more Hubs using an Atom namespaced link element with a rel attribute of "hub".
Pubsubhubbub has garnered attention because it is a pubsub protocol which is easy to implement and which operates over HTTP. Its philosophy is to replace the traditional model where blog feeds have been polled at regular intervals to detect and retrieve updates. Depending on the frequency of polling, this can take a lot of time to propagate updates to interested parties from planet aggregators to desktop readers. With a pubsub system in place, updates are not simply polled by Subscribers, they are pushed to Subscribers, elimenating any delay. For this reason, Pubsubhubbub forms part of what has been dubbed the real-time web.
The protocol does not exist in isolation. Pubsub systems have been around for a while, such as the familiar Jabber Publish-Subscribe protocol, XEP-0060, or the less well known rssCloud (described in 2001). However these have not achieved widespread adoption typically due to either their complexity, poor timing or lack of suitability for web applications. rssCloud, which was recently revived as a response to the appearance of Pubsubhubbub, has also seen its usage increase significantly though it lacks a formal specification and currently does not support Atom 1.0 feeds.
Perhaps surprisingly given its relative early age, Pubsubhubbub is already in use including in Google Reader, Feedburner, and there are plugins available for Wordpress blogs.