It starts with the assumption that individuals claim a place on the web, identified by a URL, and use this URL to identify themselves to others on the web. In the original form, it used gpg for encryption and digital signatures, XML/XPath for profile information, and also supports the sending of encrypted messages from URL to URL.
It was the original URL-based identity protocol, followed by OpenID (before the latter moved away from URLs as identifiers)
Johannes Ernst has been running it on several of his sites (including Upon2020.com(?)) since 2005.