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A new version of the classic intro from xml.com. The first version (1998) was by Tim Bray, subsequently updated by Dan Brickley (2001). The latest version is by Joshua Tauberer. It takes into account the revised specs of 2004, but more visibly makes use of Turtle syntax, which helps avoid confusion over the relationship between RDF and XML. He also nicely covers the (artificial) difference between data and metadata. Quote:

The most exciting uses of RDF aren't in encoding information about web resources, but information about and relations between things in the real world: people, places, concepts, etc.

I'm becoming increasingly convinced that the easiest practical code route into RDF for most developers will through SPARQL, but this piece gives an excellent overview of RDF itself. 

See also : Joshua's rdf:about site - it's got a range of intro material, starting with "RDF in 30 seconds"

Another quote (from Eddie Izzard, found on Joshua's homepage):

But what is an evil herbivore... I am an evil herbivore. I shall eat more leaves than I should, and other giraffes... may die. I will hide berries where no one will find them. Mwaha, mwaha, mwaha.

Hmm, :EvilHerbivore rdfs:subClassOf :Herbivore ...