From Mac Guides
RSS is a family of XML file formats for web syndication. It is currently experiencing huge growth, with growing popularity amongst blogs, news sites, Internet forums, and other sites. It also acts as the basis for podcasting.
There are a number of third party applications used exclusively for RSS. However, recently several Web browsers have also gained support for the technology. Firefox supports RSS, and as of Mac OS X 10.4, so does Safari.
- RSS at Wikipedia