Addresses like feed:http://example.org/rss.xml are typical feed protocol URL. They are used (at the moment of this writing) by feed readers/aggregators and other utilities related to feed processing. When you see them at a website, they look simmilar to the old XML icons, like in the following example:
Ok, there are a lot of interfaces now circulating TrackBack, Ping-Back, Post-It. All of these are a way of commenting on an item. The only thing missing from the mix is a way to do 'comments' themselves. That is where this specification enters. It is intended to be a roll-up of all the above specifications and to cover comments as well.