RDF -- Resource Description Framework

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Wikipedia introduction on rdf:

Resource Description Framework (RDF) is a family of specifications for a metadata model that is often implemented as an application of XML. The RDF family of specifications is maintained by the World Wide Web Consortium (W3C).

The RDF metadata model is based upon the idea of making statements about resources in the form of a subject-predicate-object expression, called a triple in RDF terminology. The subject is the resource, the "thing" being described. The predicate is a trait or aspect about that resource, and often expresses a relationship between the subject and the object. The object is the object of the relationship or value of that trait.

This mechanism for describing resources is a major component in what is proposed by the W3C's Semantic Web activity: an evolutionary stage of the World Wide Web in which automated software can store, exchange, and utilize metadata about the vast resources of the Web, in turn enabling users to deal with those resources with greater efficiency and certainty. RDF's simple data model and ability to model disparate, abstract concepts has also led to its increasing use in knowledge management applications unrelated to Semantic Web activity.

Why not just use XML?

RDF is a framework that is supposed to be used in order to describe information on the Web. XML is already widely used to markup information on the Web, so why should a new framework be developed and applied all over the Web if XML is already in place? The main answer to that question is that XML is too flexible and needs more conventions. For instance, there are numerous ways to represent the same information of a "yellow bus" using XML.

<bus color="yellow" />
<bus color="#y" /><color id="y" shade="yellow" />
<bus><color>yellow</color></bus>

By allowing the same information to be represented in various ways makes it nearly impossible for machines to parse documents in order to find relevant information. RDF tries to solve this problem by creating a standard way to interpret XML-encoded descriptions of information or resources on the Web. It accomplishes this task through RDF/XML, URIs, and the concept of subject-predicate-object expressions.

RDF/XML

RDF/XML is simply an XML document that is structured using various tags to denote the information RDF represents. The following example represents a web page resource that contains the information of a creation date. This same example can be found at in the W3C Primer on RDF (http://www.w3.org/TR/rdf-primer/#example2).

1. <?xml version="1.0"?>
2. <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.                xmlns:exterms="http://www.example.org/terms/">

4.    <rdf:Description rdf:about="http://www.example.org/index.html">
5.         <exterms:creation-date>August 16, 1999</exterms:creation-date>
6.    </rdf:Description>

7. </rdf:RDF>

We know that this is an XML document because of the XML declaration in line 1 and line 2 specifies the namespaces to be used as prefixes or QNames, for the tags in the rest of the document. Since line 2 opens a tag, and the rdf prefix is assigned to the namespace for the RDF syntax, this XML document is being used to describe RDF data. Line 4 is a RDF Description tag that is used to describe a particular resource. It has an attribute of about which specifies the exact value of the resource to be described. Line 5 specifies the predicate tag of creation-date and the value of that predicate is the object value of August 16, 1999.

RDF/XML provides the syntax that is needed to create RDF documents that represent the semantic information of a particular resource. When all of these resources are described using this syntax, it enables these resources to carry meaning that was not available before RDF/XML was created. Once meaning is given to a particular resource, machines are then able to use that information to solve problems that were once very difficult or even impossible without RDF.

RDF and RSS

RSS 1.0

Probably the most widely used example of RDF in today's version of the Web is RSS 1.0. This version of RSS stands for RDF Site Summary, but the more popular version of RSS is RSS 2.0, which stands for Really Simple Syndication. There are two versions of RSS because of a fork in the RSS development, where two independent organizations were developing the technology based on different formats. RSS 1.0 was based on RDF whereas RSS 2.0 got rid of RDF and was based just on XML. RSS 2.0 is the more popular version of RSS to use because its syntax is simpler to write and understand than RSS 1.0. The main reason why RSS 1.0 is harder to understand is because of all the QName references within its tags. This is one reason why RSS 2.0 has taken over a lot of what RSS 1.0 can accomplish. RSS 1.0 will still be around because it can represent data in the way that is needed for the Semantic Web. It follows the structure and syntax and is nothing more than RDF/XML. This makes it a very powerful technology because not only is it a way to allow a syndication feed, but it is also able to provide information for the Semantic Web.

RDF Examples

Molliza Usage of RDF: The Resource Description Framework, or RDF, is a W3C technology that we're using in Mozilla to integrate and aggregate Internet resources.

Mozilla RDF was originally used to support the Aurora/Sidebar user interface and SmartBrowsing metadata services. It's main use in Mozilla now is as a common data model and API for use in XUL-based applications.
SIMILE | Piggy Bank: Piggy Bank is an extension to the Firefox Web browser that extracts information from existing Web pages and stores it in RDF. If a Web page already links to RDF information, extraction simply means retrieving that information. Otherwise, Piggy Bank employs custom software code that untangles the "pure" information from the Web page' formatting.

Having extracted the "pure" information and stored it on your computer, Piggy Bank can now apply its own user interface to let you browse through that information independent of the original Web sites. For example, Piggy Bank can call upon Google Maps to display geographical information even if the original Web sites do not offer cartographic views of their data.

Furthermore, by storing "pure" information from different Web sites in the same data model, Piggy Bank can offer a unified view on the "pure" information regardless of its many origins.

The piece of software code that Piggy Bank uses to "purify" information within a Web page is called a screen scraper. Different screen scrapers are made for different Web pages. Piggy Bank supports an easy way to install screen scrapers, so that getting better use of a Web page's information is just a few clicks away.
RSS 1.0 - Quick Overview: RSS 1.0 ("RDF Site Summary") is an RDF Vocabulary that provides a lightweight multipurpose extensible metadata description and syndication format. In short, its a means for describing news and events so that they can be shared across the web.
RDF Homepage: RDF Homepage creates personal homepages based on the RDF data you supply.
NEPOMUK - The Social Semantic Desktop: NEPOMUK stands for Networked Environment for Personalized, Ontology-based Management of Unified Knowledge and its goal is to realize and deploy a comprehensive solution - methods, data structures, and a set of tools - for extending the personal computer into a collaborative environment, which improves the state of art in online collaboration and personal data management and augments the intellect of people by providing and organizing information created by single or group efforts.
ARC RDF Store: ARC RDF Store is a SPARQL-enabled RDF storage system optimized for shared, hosted (or otherwise performance-limited) Web environments. It can be integrated in PHP/MySQL-driven frameworks and tries to lower the barrier to RDF-based Web application development.

W3C Semantic Web Announcements

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Squidoo Provided Links

W3C Resource Description Framework: Official pages from the World Wide Web Consortium, includes the specification, resources and news, and a links collection.
RDF Resources: Links to various documentation of RDF sources.
RDF Logic Mailing List: Provides a forum for technical discussion concerning the design of logic-based languages for use on the Web.
Hyphen.info: Repository of RDF hierarchies.
RDF Interest Group: The main discussion forum for RDF developers and users.
RDF-DEV : A Site for RDF Developers: Resources database containing links to guides, papers, tools, and standards in addition to the archives of the (now closed) RDF-DEV mailing list.
Dave Beckett's RDF Resource Guide: An extensive and annotated collection of links to RDF documents and software, updated frequently.
Extensibility: Details on this XML, Semantic web, and web services metablog and news aggregator.
SchemaWeb: Repository for RDF schemas expressed in the RDFS, OWL, and DAML+OIL schema languages.
Introduction to RSS News Feeds: Find out how to create and use RSS files and learn what they can do for you. See why companies like Netscape, Userland, and Moreover use RSS to distribute and syndicate article summaries and headlines. This article includes sample code that demonstrates elements of an RSS file, plus a Perl example using the module XML::RSS.
PML: Proof Markup Language is used to build OWL documents representing both proofs and proof provenance information.