In the beginning was RSS 0.x. It was originally intended to be based on RDF. Compromises were made and it ended up dropping the RDF. The spec. wasn’t very good and had several ambiguities.

Some people forged ahead with a cleaned up RDF-based version and called it RSS 1.0, while other people went ahead with the ambiguities but without RDF and called it RSS 2.0. The person publishing RSS 2.0 considered it finished and refused to update it. There was drama.

A bunch of people decided that there was too much to clean up from within that mess and started a new format, Atom. This ended up being a much better spec. with an official RFC, but at this point everybody was calling any type of feed “RSS”, even if it was Atom.

If you have the choice, you should pick Atom.