Real Simple Syndication (RSS) is a format for syndicating content. This content can be from any source, news, blogs, podcast, etc. This format allows the content generator to produce an index of sorts, where they can briefly describe their content and include links to the full story.
RSS feeds can be read several different ways. RSS aggregators allow you to subscribe to many feeds and will indicate when the feed has been updated. Feeds are meant to be dynamic, and include modification date to indicate if there is new content for the reader.
RSS Readers are used to view the RSS feeds. There are many RSS readers with some of the largest being MyYahoo!, Google, MyAOL, Pluck, MyMSN, Newsgator, Netvibes, Pageflakes, Bloglines and Rojo.
These readers allow you to subscribe to the RSS feeds that interest you. This gives you a custom set of content for you to read that matches your interests.
More information and examples of RSS feeds.