RSS is an acronym for a a couple of terms (dunno which one's official)... "Rich Site Summary" and "Really Simple Syndication". This is an xml standard for content aggregation. The idea is simple. Instead of going to different sites to see what new content is available, you get notified of what's new. This is made possible with the agreed upon format which is RSS. The idea has been there for a while with both Netscape and Microsoft coming up with their own formats, but it didn't take off until the RSS standard came about.
RSS can be used for any sort of content. Probably the most popular use of RSS will be from news sites as that content is ALWAYS being updated. But it is used for all sorts of stuff including comics and blogs. There is a rival format called "Atom" and Google has gotten behind that, so the Blogger feeds are in the "Atom" format. Infact codeword has a blog feed... http://codeword.blogspot.com/atom.xml.
You would need some sort of feed aggregator application to make use of the feeds. You give the app URL's to the site feeds (like the codeword feed). If you look at news sites like BBC or Slashdot, they will have a red graphic with the word "XML" on it. That's the feed URL for the site. As the content on the site is updated, the RSS document is updated. The feed aggregator app periodically checks this document and informs you that the content has been updated. So as an example, instead of visiting http://codeword.blogspot.com everyday to check if something new has been posted, the feed aggregator app will tell you. I use Abilon. It's very basic and does everything an aggregator would need to do.
It's such a simple concept. The only thing that was needed was a standard and now with RSS/Atom they have it. I really feel like it's a "killer app".