What is RSS? To understand the answer to that question, you need to know something about XML, for this is the format of RSS.
XML stands for extensible markup language and it is simply a set of rules for encoding documents. RSS stands for really simple syndication, and it is a type of news feed in XML format. (Other tools that make use of XML, by the way, are office productivity tools such as Microsoft Office, OpenOffice.org, and Apple's iWork.)
Summary:
An RSS reader allows you to subscribe to feeds for any topic under the sun, so that you can easily browse through items of interest to you. It pulls information to you rather than you having to go out to it.