There are many interpretations for RSS, this one is from the Essential Blogging

Rich Site Summary (RSS) is a data format that allows computers to exchange files containing summaries of stories. Each story typically has a title, location, and possibly a brief synopsis. RSS is extremely simple, and is expressed using standards-defined Extensible Markup Language (XML). Thousands of blogs and other web sites all over the Net produce RSS files describing their content.

The cool thing about RSS is that it’s easy to write a program to produce or manipulate the data in an RSS document. RSS is so simple to generate and manipulate that hundreds of programmers have written tools to exploit the thousands of RSS feeds on the Internet.