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RSS is most commonly defined as "Really Simple Syndication" but sometimes "Rich Site Summary". It is a way for a webpage, most commonly in the form of a blog to syndicate either as a partial or full representation of the pages content.
RSS can allow you to quickly check your favorite blogs or often updated sites in an easy to digest format. It also allows a publisher to aggregate their content and repurpose it into other platforms and websites.
Originally, feeds were pulled from the source into desktop readers like Feed Deamon and others. More recently feeds are pulled into browsers like Firefox and Internet Explorer as well as online readers such as Google Reader and My Yahoo.
Originally heralded as the great email replacer, I personally find RSS more interesting as a way that websites syndicate into other pages than as a way to independently read content. My attitude is if I really am tired of the email I unsubscribe or create rules whereby my categorized email goes into folders. Of course free email like Gmail can always keep my commercial email separated.