The Resource Description Framework (or RDF) is a generic mechanism for encoding MetaData -a framework to gave explicit meaning to infomation- in order to support the processing and exchange of knowledge. It uses XML to express an ontology, i.e., the meaning of each part and its relationships.
References:
http://w3.org/RDF/ (specs, primers, tools, etc.).
http://w3.org/TR/rdf-primer/ (primer)
RDF and other related technologies are fundamental for the SemanticWeb (where you'll find more references).
The ESW wiki is dedicated to RDF and related technologies.