Linked Data

The Semantic Web isn't just about putting data on the web. It is about making links, so that a person or machine can explore the web of data.  With linked data, when you have some of it, you can find other, related, data.
Like the web of hypertext, the web of data is constructed with documents on the web. However,  unlike the web of hypertext,  where links are relationships anchors in hypertext documents written in HTML, for data they links  between arbitrary things described by RDF,.  The URIs identify any kind of object or  concept.   But for HTML or RDF, the same expectations apply to make the web grow:

1. Use URIs as names for things
   
2. Use HTTP URIs so that people can look up those names.
   
3. When someone looks up a URI, provide useful information, using the standards (RDF*, SPARQL)
   
4. Include links to other URIs. so that they can discover more things.
   

Simple.  In fact, though, a surprising amount of data isn't linked in 2006, because of problems with one or more of the steps.  This article discusses solutions to these problems, details of implementation, and factors affecting choices about how you publish your data.

The four rules

I'll refer to the steps above as rules, but they are expectations of behavior.  Breaking them does not destroy anything, but misses an opportunity to make  data interconnected.  This in turn limits the ways it can later be reused in unexpected ways.  It is the unexpected re-use of information which is the value added by the web.
The first rule, to identify things with URIs,  is pretty much understood by most people doing semantic web technology.  If it doesn't use the universal URI set of symbols, we don't call it Semantic Web.

The second rule, to use HTTP URIs,  is also widely understood.  The only deviation has been, since the web started,  a constant tendency for people to invent new URI schemes (and sub-schemes within the urn: scheme)  such as LSIDs and handles and XRIs and DOIs and so on, for various reasons.  Typically, these involve not wanting to commit to the established Domain Name System (DNS) for delegation of authority but to construct something under separate control.   Sometimes it has to do with not understanding that HTTP URIs are names (not addresses) and that HTTP name lookup is a complex, powerful and evolving set of standards. This issue discussed at length elsewhere, and time does not allow us to delve into it here. [ @@ref TAG finding, etc])
The third rule, that one should serve information on the web against a URI, is, in 2006, well followed for most ontologies, but, for some reason, not for some major datasets.  One can,  in general,  look up the properties and classes one finds in data, and get information from the RDF, RDFS, and OWL ontologies including the relationships between the terms in the ontology.
The basic format here for RDF/XML, with its popular alternative serialization N3 (or Turtle). Large datasets provide a SPARQL query service, but the basic linked data should br provided as well.
Many research and evaluation projects in the few years of the Semantic Web technologies produced ontologies, and significant data stores, but the data, if available at all, is buried in a zip archive somewhere, rather than being accessible on the web as linked data.  The Biopax project, the CSAktive data on computer science research people and projects were two examples. [The CSAktive data is now (2007) available as linked data]
There is also a large and increasing amount of URIs of non-ontology data which can be looked up.  Semantic wikis are one example. The "Friend of a friend" (FOAF) and Description of a Project (DOAP) ontologies are used to build social networks across the web.    Typical social network portals do not provide links to other sites, nor expose their data in a standard form.
LiveJournal and Opera Community are two portal web sites which do in fact publish their data in RDF on the web.   (Plaxo has a trail scheme, and I'm not sure whether they support knows links). This means that I can write in my FOAF file that I know Håkon Lie by using his URI in the Opera Community data, and a person or machine browsing that data can then follow that link and find all his friends. [Update:] Also, the Opera Community site allows you to register the RDF URI for yourelf on another site. This means that public data about you from different sites can be linked together into one web, and a person or machine starting with your Opera identity can find the others.
The fourth rule, to make links elsewhere,  is necessary to connect the data we have into a web, a serious, unbounded web in which one can find al kinds of things,  just as on the hypertext web we have managed to build.
In hypertext web sites it is considered generally rather bad etiquette not to link to related external material.  The value of your own information is very much a function of what it links to, as well as the inherent value of the information within the web page.  So it is also in the Semantic Web.
So let's look at the ways of linking data, starting with the simplest way of making a link.