It's a tongue twister, "Pub Sub Hub Bub". PSHB is a protocol designed to make pushing updates to feeds simple for the publisher and subscriber by hiding as much complexity in a hub in the middle
The PubSubHubBub main site contains a link to the latest specification. Here's the 0.3 specification.
I'm in the process of making some websequence diagrams to illustrate how things work.
I've removed some of the details to give a overview of the "happy path"
Click the diagrams to be taken to the websequence.com source code. Free to Share, Remix. Please attribute me and contribute back changes in the comments for this page.
 PubSubHubBub Overview Sequence Diagram by Oliver Bye is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. DiscoveryThe subscriber discovers a Feed. The content of the feed declare support for PSHB.
The subscriber informs the hub of their call back URL. This must be a valid URL, visible to the Hub (e.g. not behind a firewall).
The Feed publsher updates their feed content then informs the Hub that the content has changed. The Hub fetches the updated feed and computes the changes. The Hub then distributes the content to all the subscribers.
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