RSS has been around since the 80's. The standards have become fragmented into different branches the specifications for each version of RSS is below.

RSS Versions (2.0 series)

RSS 2.0	current version Really Simple Syndication
RSS 0.94	Really Simple Syndication
RSS 0.92	Really Simple Syndication
RSS 0.91	Really Simple Syndication

Atom Versions (0.3 series)

Atom v.03

RDF Versions (series)

RSS v1.0

Rich Site Summuary Specification (RDF)