A feed is frequently updated content that is published on a web site or blog. The purpose of RSS is to allow the content to be distributed. If I have a standard web site or blog, I am dependent on you coming to my web site to read my new information. A feed allows me to send information to you. You control the information you receive by subscribing to the feed. A feed is typically a small piece of information on a specific topic. Instead of sending a newsletter to ALL of my clients about ALL of the software I work with, a feed allows me to send just Salesforce articles to just Salesforce users. I would compare RSS feeds to a clipping service. You’re saying, “These are the topics I’m interested in. Search them out and deliver them to my reader.”
Most subscriptions are free. A subscription will automatically pull new posts from the RSS feed and let you view them in your reader. Most readers give you the ability to tag posts for later searches. You can build your own library of information about your favorite topics.
Take a look at these sites which offer RSS feeds:
http://www.webmd.com/ – medical information site