Facts are encoded as triples:

• a URI (something that can be named uniquely, e.g. a webpage location, a book title via ISBN).
• A property name such as surname, phoneNumber.
• A value, either a literal or a pointer to where the information can be found, e.g. the surname or the phoneNumber.

• a URI of a person.
• A property name such, e.g siblingof, parentof, childof, spouseof
• a URI of another person.

In the XML representation, the properties may manifest as either tags or attributes just to keep you on your toes.

There are many competing ontologies and ontology schemas. I gather the hope is it will eventually narrow down to a few ontologies in each field and that someone will figure out how to make them interoperable. Ontology schemes such as DAML+OIL also allow you to describe logical relations, e.g. who eats who in a ontology of animals and plants. Another is OWL.

The other thing RDF tackles is the issue of trust. Who said such and such a fact was so. It uses digital signatures.

RDF provides FOAF with a way to mix together different descriptive vocabularies in a consistent way. Vocabularies can be created by different communites and groups as appropriate and mixed together as required, without needing any centralised agreement on how terms from different vocabularies should be written down in the XML.