RSS allows you to subscribe to sites with frequent updates such as news articles and blogs and get them sent to you instead of having to go to each site to get them. You read these files in a program called an aggregator, which collects "feeds" from various websites and provides them to you in a simple form.
Web: FeedlooksRead full content, from right within the app.
Mac OS X: NetNewsWireThis elegant Mac-like aggregator is easy to use and powerful.
Windows: SharpReaderSimple, but gets the job done.
Linux: LifereaA simple Gtk newsreader.