Wikipedia defines RSS as "a family of Web feed formats used to publish frequently updated content such as blog entries, news headlines or podcasts.  An RSS document, which is called a "feed", "web feed", or "channel", contains either a summary of content from an associated web site or the full text. RSS makes it possible for people to keep up with their favorite web sites in an automated manner that's easier than checking them manually."

RSS in Plain English by CommonCraft

RSS, also known as Really Simple Syndication, is a means of sharing content without requiring the reader to constantly visit a Web site to see what's new.  The reader subscribes to the site as he/she would a magazine.  This content now comes to your reader for you to read.

Bloglines

Google Reader

Pageflakes

Netvibes

Yahoo Reader

NewsGator