Really Simple Syndication (RSS) is an XML format designed for sharing news and other Web content. Think of it as a way to subscribe to websites, and elements of websites, that you'd like to receive on an ongoing basis.
Of course, you'll need an RSS reader to view these feeds. They typically come in two varieties: Desktop and Web-based. Desktop aggregators are applications that reside on your computer and store and present all the feed content locally. Web-based readers are aggregators that you view through your browser and the feeds and content are stored on the systems hosting the application.
If you need more details on RSS, check out this article.