Really Simple Syndication (RSS) is a lightweight XML format designed for sharing headlines and other Web content. Think of it as a distributable "What's New" for your site. But it's not just for news. Pretty much anything that can be broken down into discrete items can be syndicated via RSS: the "recent changes" page of a wiki, a changelog of CVS checkins, even the revision history of a book. Once information about each item is in RSS format, an RSS-aware program can check the feed for changes and react to the changes in an appropriate way. Attention, your browser may not support RSS/xml.

RSS and Mac OS X:
http://www.apple.com/macosx/features/safari