RSS (really simple syndication OR remote site syndication) is a technology for distributing your content in a way that makes it easier for your customers to get the messages they want from you.
Note: I said "that they want." Not "that you want to deliver."
RSS allows us to get closer to the practices of Permission Marketing that Seth Godin preaches. It's a way to respect customer's time and give them the ability to subscribe to your content via web browser, email or portal page (like Google). This differs from email in that your content gets delivered automatically to a "channel" or folder in your computer program.
Here's what customers can do if you add RSS capabilities (called a RSS feed) to your web site:
Your information appears directly on their Google home page with a link back to the original article on your web site or blog.
Want more information? Bill Babcock wrote a short primer on the topic on his blog.