PubSubHubBub is realtime RSS
PubSubHubBub is a new standard that allows a site owner to push new content directly into Google's index. With RSS, Google (or any other search engine) still needs to review it regularly to see if there is any new content. With PubSubHubBub, this is not the case, giving the content owner: there is new content. This makes it possible to see real-time content in the Google search index. The content provider still sends out the normal RSS feed, but also notifies a hub so that it can send it directly to the recipients. It is an open source code project from Google. One of the biggest blog platforms (WordPress) uses PubSubHubBub which means that the websites hosted on WordPress.com are instantly indexable.