Basically RSS is a subset of XML which is used to allow people to syndicate information feeds from their sites.
Where things get a little more complex is that it is a parallel standard. It was developed by Netscape and the first standard was RSS 0.91. However, it was felt that this had strayed from the RDF roots and so the 1.x standard was developed. Both these standards are developing in parallel and RSS 1.x is not a replacement for RSS 0.9x. RSS developers hope to freeze development on this standard with the announcement of RSS 2.01 (confusing matters rather).
In summary:
RSS 0.9x/2.x (where RSS means Really Simple Syndication) is as its name suggests a very simple way of sending the information.
RSS 1.x (where RSS means RDF Site Summary) is more complex and offers the option of using custom made modules to cope with different needs.
