RSS (Rich Site Summary) is a format for distributing news announcements and content over the internet. RSS feeds are widely used by news and commerce sites to alert visitors when something on a site has changed, such as articles or prices. A typical RSS feed includes a linked headline and short descriptive blurb, such as "Perl 6 released to a swarthy crowd of expectant fans."
You'll need an RSS reader before you can view RSS feeds in human readable format, and you'll need to subscribe to feeds, in the same way you need to subscribe to newsletters, before you can receive them. Once you've subscribed to an RSS feed you no longer have to visit a website to know if something has changed; the change will come to you.
An RSS reader is a piece of software that converts the bits and bytes that comprise a "feed" and converts it to human readable text. There are many RSS readers to choose from, both free and for a fee. One of many lists of RSS readers can be found here, which is conveniently organized by the kind of computer you have.
You've probably noticed those ubiquitous orange RSS and ATOM buttons on your favorite websites. If not, they look like this:
Any site that displays this icon offers feeds you can subscribe to.
There are two easy ways to subscribe to RSS feeds. The first is to type the url of the page the feed buttons appear on into your reader. For example, you can type "oreilly.com" into your reader, preview the feed, and then click the "subscribe" button located in your reader. Once subscribed, notification of changes and updates are sent to you. The second method is to right-click one of the feed links to the right. In Firefox, select "Copy Link Location." In IE, select "Copy Shortcut." In Opera, select "Copy Link Address." Once you've copied the link, open your preferred reader and paste the feed URL into the appropriate "Subscribe to Feed" text box. Either method gets you subscribed to the desired feeds.
Be aware that many sites have several feeds, and you will have to subscribe to each feed. Many sites also have pages, like this one, that consolidate all the feeds offered.
You can learn more about RSS feeds and the associated technology from the following resources: