
There are quite a few definitions, but you should try google...
It's a code standard for snippets of information. Generally they contain a headline and a link to the article - more advanced versions also contain a description text.
Take a look at
http://clubtotal.free.fr/ and scroll down the page. This guy, Maxwish, shows RSS-feeds on his frontpage (which is one way to show them), all the boxes with news are RSS-feeds (Security news from Bugtraq, Software news from Betanews.com etc).
I pull RSS-news into my Instant Messenger (Miranda IM - also has my ICQ, MSN, mailchecks and IRC), and either get a 'message' (just like on messenger) and/or a little popup showing the RSS feed.
There are numerous programs (and scripts for homepages) for showing RSS-news - you'll also find it under the name XML-feed, newsfeed, RDF. It's a VERY loose standard, unfortunately - but it's getting better.