Linked Data, Web of Data, and the Semantic Web
I have been wanting to write a post on the relationship of Linked Data and the Web of Data to the Semantic Web, but even now I am still struggling to get a secure handle on the distinctions between these three related concepts. Meanwhile, I stumbled across a relevant blog post by Tom Heath on the topic entitled "Linked Data? Web of Data? Semantic Web? WTF?" It's difficult to get a hard-core representative summary, but a semi-reasonable approximation is:
... in common usage Linked Data refers to the principles set out by Tim Berners-Lee in 2006.
So if we link data together using Web technologies, and according to these principles, the result is a Web of data. Personally I use the term Web of data largely interchangeably with the term Semantic Web, although not everyone in the Semantic Web world would agree with this. The precise term I use depends on the audience. With Semantic Web geeks I say Semantic Web, with others I tend to say Web of data -- it's not about rebranding, it's about using terms that make sense to your audience, and Web of data speaks to people much more clearly than Semantic Web. Similarly, Linked Data isn't about rebranding the Semantic Web, it's about clarifying its fundamentals.
Tim Berners-Lee said several times last year, in public, that "Linked Data is the Semantic Web done right" (e.g. see these slides from Linked Data Planet in New York), and who am I to argue, it's his vision.
I am still not prepared to write the definitive post on this topic, but here is the gist of my current research:
- W3C offers a number of Semantic Web technologies, including XML, XML Schema, RDF, RDFS, RDFa, OWL, SPARQL, XSLT, and others.
- The Semantic Web is the vision of the World Wide Web that utilizes the Semantic Web Technologies, particularly RDF as its core.
- Any application can utilize any one or more of the Semantic Web technologies.
- Mere use of Semantic Web technologies does not by itself indicate that the application is a Semantic Web application.
- A Semantic Web application is first and foremost a Web application, typically accessible on the World Wide Web, that utilizes Semantic Web technologies, and specifically uses RDF (or RDFa) for making statements about (Semantic) Web resources.
- A Semantic Web application might typically include a more traditional Web application (e.g., HTML) combined with underlying Semantic Web resources.
- Web of Data is simply a casual synonym for the Semantic Web that emphasizes that like the original, non-Semantic Web, the Semantic Web consists of an interconnected Web of resources, but they are data resources described at their core using RDF (or RDFa) rather than merely presentation resources (HTML web pages.)
- Linked Data is not introducing any new technologies, but is simply a collection of principles that emphasize that the Semantic Web (or Web of Data) has much greater utility to its users when data resources tend to refer to other, somewhat related data resources that may not necessarily be directly required by the local Semantic Web application.
- Put simply, Linked Data enables the user (or computational user agent) to navigate between Semantic Web applications (data resources).
- Even a proprietary application that uses Semantic Web technologies may also utilize resources (e.g., vocabularies or schemas) from elsewhere in the Semantic Web, but the real test of whether an application is a true Semantic Web application is whether other applications in turn reference it. It is this expanding chain of referencing to produce an ever-expanding and ever more-heavily interconnected Web that gives the Semantic Web its true "webbiness", not the mere use of the underlying Semantic Web technologies by themselves.
That model is not entirely accurate, but I think it's a good start. I need to include mention of HTTP and URIs; they are not unique to the Semantic Web, but are essential.