RSS and ATOM are methods of syndicating web content.  RSS has quickly grown in popularity, and will be natively support in the next version of Windows.  The benefits of syndicating your content is that users that wish to subscribe to your websites content will automatically be informed when new content is added.

FireFox already has the functionality built in to bookmark live RSS feeds, and RSS support is expected to be a large part of the improvements in the next version of Internet Explorer (IE 7).

Currently RSS 2.0 is the most popular syndication format, however that may change in the near future with ATOM taking it's place.