A feed is the most convenient way to be notified when a site changes. Instead of checking RecentChanges every day to find out what's new, the site will send you an alert when it changes, which you can read in your news aggregator. (To receive alerts via email, you want MailPosts.) If you want to find feeds on other websites, look for terms or icons like "XML", "RSS", "Atom", "Subscribe", "Syndicate", "Bloglines", "Updates", or "News". Feeds are still fairly new, so the terms aren't standardized yet.
Feeds in PmWiki build on the WikiTrails idea. The page that is browsed with
?action=rss is treated as a WikiTrail. In the RSS feed produced, the trail page is the channel, and each item on the trail is a news item. So, you can use any RecentChanges page, or Site.AllRecentChanges as source WikiTrail for each group and the whole site. And so long as they can edit pages, visitors can have custom feeds without asking the administrator for help. All they have to do is create a page with a wiki trail of the pages that they want update notices for.
More technically, a feed is a document written in the XML language, and following a syndication format like RSS or Atom. It will list the last N changes (N being whatever number the feed's maker decided it should be), and will give headlines. If the feed's maker decided to, there might also be summaries of the new content, or even the full text of the changed page.
?action=rssonto the end of the URL you want to subscribe to. So, if you wanted to subscribe to pmwiki.org's AllRecentChanges, you'd take its address http://pmwiki.org/wiki/Site/AllRecentChanges, and add
?action=rssto that. The result, http://pmwiki.org/wiki/Site/AllRecentChanges?action=rss, is the feed address for your news aggregator.
Note: In addition to the support bundled with PmWiki, there is a Cookbook:RssImproved recipe in the cookbook, which supports RSS 2.0, enclosures/podcasting and improved customizability. - XES
somewhere in your local/config.php and supplying
?action=rss in the URL to a RSS aggregator (
?action=rdf will output RSS v1).
Glad you asked! Back when the whole idea of RSS was invented it stood for RDF Site Summary where RDF stands for Resource Description Framework. Basically RDF was a way to describe meta-information about content using XML. The only problem with RSS back then was that it had very little to do with RDF (RDF headers were output, but the rest of the document was different XML than what RDF defined). So, anyway, this idea of RSS evolved for a time and gave birth to RSS v1.
Sometime later Dave Winer decided to redo the RSS spec and gave the acronym a new meaning -- Really Simple Syndication. Thus was born RSS v2 which was quite different from RSS v1 but still trying to accomplish the same goals.