Wikipedia provides the following description:
RSS is a family of Web feed formats used to publish frequently updated works such as blog entries, news headlines, audio, and video in a standardized format. An RSS document (which is called a "feed," "web feed," or "channel") includes full or summarized text plus metadata such as publishing dates and authorship. Web feeds benefit publishers by letting them syndicate content quickly and automatically. They benefit readers who want to subscribe to timely updates from favored websites or to aggregate feeds from many sites into one place.
This video, courtesy of CommonCraft's "plain English" series, provides a simple explaination of RSS feeds.
