RSS is an acronym
for Rich Site Summary, an XML
format for distributing news headlines on the Web, also known as syndication
First started by Netscape as part of the My Netscape site, it expanded through Dave Winer and Userland. RSS started off in an RDF format, but RSS 0.91 (the current version) is not, which leads to some annoying issues when trying to parse different pages.
Virtually every news site and weblog uses RSS for syndicating content. It would actually make for a really nice Everything feature. The most useful site for RSS information is probably http://www.oreillynet.com/topics/rss/rss, but http://www.webtechniques.com/archives/2000/02/eisenzopf/ serves as a nice introduction.