The Semantic Web In Breadth - A comprehensive introduction into RDF and Semantic Web technologies by Aaron Swartz - http://logicerror.com/semanticWeb-long
What is RDF? - A good introduction to RDF (updated version of RDF and Metadata) by Tim Bray - http://www.xml.com/pub/a/2001/01/24/rdf.html
RDF Tutorial - An introduction to RDF in HTML, PS and PDF forms. - http://www710.univ-lyon1.fr/~champin/rdf-tutorial/
RDF and Metadata. - By Tim Bray. - http://www.xml.com/pub/a/98/06/rdf.html