This is not a technically correct description but, in my mind, RSS is basically a system by which you can subscribe to a feed from a website. You don't have to go to the website and check whether it has been updated, your feed reader will tell you it has been updated and provide direct links to the new content.
I found the BBC's introduction to RSS very clear on this.
Your feed reader could be a bit like Microsoft Outlook (a downloaded program on your computer) or it could be a feature on your internet browser (Firefox already does this and I think the next version of Internet Explorer will). However, I prefer service called Bloglines because it is web based, so I can access it from anywhere. Google has a similar reader. The simplest introduction to RSS is signing up to a www.my.yahoo.com and sticking a few rss feeds onto your homepage. Although this method means it is difficult to process large numbers of web feeds, it is more like a traditional newspaper in that it feeds news onto a laid out page.
Basically, in all of these programs, you click on a link (or input a url) to an RSS feed and it somehow zaps it into your RSS reader and forever more you have a feed available on your reader monitoring updates to that site. This site, for instance, has a RSS feed here and a Japanese news feed here.