The Resource Description Framework (RDF) provides a simple way to describe resources on the web. It is a directed, labeled graph data format for representing information in the Web.
Knowledge is represented as assertions (or triples) of the form:
URI — property — value
where 'URI' is the URI of the thing being described, 'property' is (the URI of) a property, and 'value' is the value that that property can take, either another URI, a literal string, or a chunk of XML.
See RDF in Drupal: Super-Simple Primer http://drupaleasy.com/blogs/ultimike/2009/06/rdf-drupal-super-simple-primer for a quick overview and the RDF Primer http://www.w3.org/TR/rdf-primer/ for an in depth tutorial.
W3C RDF Validation Service http://www.w3.org/RDF/Validator/