RDF is short for Resource Description Framework. XML is short for eXtensible Markup Language. That was the easy part. RSS has a number of competing versions, one of which is RSS 1.0. It is based on RDF, which is a broader web standard for specifying information about resources (such as a web page). RDF is a specific form of XML. RSS 2.0 and RSS 0.9x are not based on RDF. (See
What is RSS from xml.com for more on this). So all RDF is XML, but not all XML is RDF. And RSS could be either.
All this makes no difference to users -- readers do take either.
I think most sites choose which format to provide based on which software they're running. The additional information provided in each format isn't of huge utility in most cases, IMHO.