RDF is the tool for adding semantics (meaning) to a Content Object? in web pages.

This meaning can be discovered by parsers and inference engines, so it goes beyond the added meaning provided by human understandable XML tags. It allows computer programs to discover the meaning.

RDF is used for library catalogs, for large directories, media asset repositories, and for personal media collections like books, CDs, and digital images. RDF is most often transferred between computers using XML syntax.

XML began as a markup language (somewhere between simple HTML? and sophisticated SGML?) but is now most often used as a data exchange standard.

Where XML has the architecture of a hierarchical tree, RDF consists of sets of triples each of which has the form of a Statement like Subject - Predicate - Object.

Collections of RDF triples form an RDF Vocabulary and when they cover a field of knowledge they constitute an Ontology.

Note that the RDF is the set of abstract triples and not any particular representation of them like the XML that transfers them between machines.

See also RDF Site Summary (RSS).