Userland says RSS stands for "Really Simple Syndication". MSDN Magazine(June 2003) says "Rich Site Summary". My question is, which one is it?[Jeff Julian's Blog]
From what I understand, it doesn't officially stand for anything.
"One of the strange things about the weblogging world is that there are two XML formats with the same initials: Really Simple Syndication, and RDF Site Summary. [RSS vs. RSS]
Another name I've heard is Rich Site Summary, as you mentioned.
dive into mark has a pretty good piece on the history of the rss fork. I also had a good link awhile back that described the relationship(s) at a high level, I'll see if I can dig that up too.