RSS, or "Really Simple Syndication," according to Dave Pollard at Salon, is "a mechanism for publishing (syndicating) and subscribing to recent additions to any website -- usually a news site or a weblog. ... The RSS software tracks new posts on your blog, or your favourite news site, packages them all up in a standard format, called a feed, and sends them to anyone who has 'subscribed' to them. All your RSS feeds are integrated together, creating a kind of personalized newspaper. Some RSS feeds contain the entire article, while others offer just the headline, or the headline with the first part of the article, with a link to the full article." 
RSS is "an Internet standard just like HTML (i.e. the web) -- with one key difference -- delivery. When you use the web, you're constantly browsing, searching ... seeking information. ... With RSS new information -- information that you select and information you can turn off at any time -- is automatically delivered directly to you. This information is called a Feed since it feeds direct to you. ... Within an RSS feed there can be text, links pictures and more just as you find on the web. The different 'chunks' of information within an RSS feed are called posts." 
"Any website, weblog or even any ecommerce site can provide information to you as an RSS feed. Even Amazon is publishing RSS feeds now, as are the New York Times, Yahoo News, Harvard, Microsoft and millions of others." 
"Just has you have a browser for the web, there are browsers for RSS. These browsers are called News Aggregators or 'Aggies' because what they do is aggregate, i.e. gather and present new information." 
Mark Pilgrim at xml.com explains that RSS is "a format for syndicating news and the content of news-like sites, including major news sites like Wired, news-oriented community sites like Slashdot, and personal weblogs. But it's not just for news. Pretty much anything that can be broken down into discrete items can be syndicated via RSS: the 'recent changes' page of a wiki, a changelog of CVS checkins, even the revision history of a book. Once information about each item is in RSS format, an RSS-aware program can check the feed for changes and react to the changes in an appropriate way. 
"RSS-aware programs called news aggregators are popular in the weblogging community. Many weblogs make content available in RSS. A news aggregator can help you keep up with all your favorite weblogs by checking their RSS feeds and displaying new items from each of them." 
See remainder of Pilgrim's article "What is RSS?" for further explanation.