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Resource Description Framework

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Resource Description Framework (RDF) is the specification for a metadata model (often implemented as an application of XML) that is maintained by the World Wide Web Consortium. This model is based upon the idea of making statements about resources in the form of a subject-predicate-object expression (in RDF terms, called a triple). The subject is essentially the resource, the "thing", being described. The predicate is what trait or aspect about that resource that is being described. And the object is what the value of that trait is.

The terminology is taken from logic and linguistics where subject-predicate or subject-predicate-object structures have very simlar but definitely distinct meanings. For RDF, there is again a similar but distinct meaning that might be best described by example.

In the statement 'New York's postal abbreviation is NY', 'New York' would be the RDF subject. 'postal abbreviation' the RDF predicate and 'NY' the RDF object.

As far as encoding this into actual RDF triples, simply stating 'New York's postal abbreviation is NY' is not enough. In a like manner, to say that the title of this page is "RDF" and its publisher is "Wikipedia" would be two statements that would need more to them to be valid RDF statments. In the [N-Triples] form of RDF, these statements might look like:

 <http://en.wikipedia.org/RDF> <http://purl.org/dc/elements/1.1/title> "RDF" .
 <http://en.wikipedia.org/RDF> <http://purl.org/dc/elements/1.1/publisher> "Wikipedia" .

and might be expressed in RDF/XML as:

<rdf:RDF
 xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
 xmlns:dc="http://purl.org/dc/elements/1.1/">
<rdf:Description rdf:about="http://en.wikipedia.org/RDF">
  <dc:title>RDF</dc:title>
  <dc:publisher>Wikipedia</dc:publisher>
</rdf:Description>
</rdf:RDF>

Why this would be useful in any way is probably not apparent, but the crucial thing to understand is that while "the title of this page is RDF" might have meaning to a human, it means nothing to a computer. The purpose of RDF is to provide an encoding and interpretation mechanism so that resources can be described in a way that particular software can understand it.

Both versions of the statements above are wordy because one requirement for a resource (as a subject or a predicate) is that they be unique. It is obvious why the subject resource should be unique. It says what exact resource is being described. The predicate needs to be unique so that the idea of Title or Publisher will not be ambiguous to software working with the description. If the software recognizes what [1] (http://purl.org/dc/elements/1.1/title) (a specific definition for a the concept of a title established by the Dublin Core Metadata Initiative), it will also know that this title is different from a land title or an honarary title or just the letters t-i-t-l-e put together.

This mechanism for describing resources is a major component for what is proposed by the W3Cs Semantic Web activity to be the next evolutionary stage of the Web, enabling automated software to work with metadata so that users can deal with vast resources of the Web more efficiently and with more certainty.

RDF implementations:

RDF Site Summary - this is RSS 1.x (Really Simple Syndication, aka RSS 0.9x and 2.x, isn't based on RDF).
FOAF - designed to describe people, their interests and interconnections.