Semantic web, RDF in D7, what is this all about?

(Note: I am more curious than knowledgeable in this field. This BoF is aiming to be more a knowledge sharing session rather than a formal presentation)

I keep meeting people curious about all these semantic web options in Drupal 7, but not many people seem to understand the whole thing or to have done significant features with it.

Let's get together and see if we can all understand a bit more what it's all about : RDF and RDFa, SPARQL, ontologies, etc.