Here's the Wikipedia definition of RSS:

RSS is a family of web feed formats specified in XML (a generic specification for data formats) and used for Web syndication. RSS delivers its information as an XML file called an "RSS feed", "webfeed", "RSS stream", or "RSS channel". These RSS feeds provide a way for users to passively receive newly released content (such as text, web pages, sound files, or other media); this might be the full content itself or just a link to it, possibly with a summary or other metadata (data describing the content).

RSS feeds are operated by many news web sites, weblogs, schools, and podcasters.

"RSS" can stand for any of the following phrases:

• Really Simple Syndication (RSS 2.0)
• Rich Site Summary (RSS 0.91, RSS 1.0)
• RDF Site Summary (RSS 0.9 and 1.0)