Linked data
In computing, linked data (often capitalized as Linked Data) is structured data which is interlinked with other data so it becomes more useful through semantic queries. It builds upon standard Web technologies such as HTTP, RDF and URIs, but rather than using them to serve web pages only for human readers, it extends them to share information in a way that can be read automatically by computers. Part of the vision of linked data is for the Internet to become a global database.

Tim Berners-Lee, director of the World Wide Web Consortium (W3C), coined the term in a 2006 design note about the Semantic Web project.

Linked data may also be open data, in which case it is usually described as linked open data (LOD).

Principles
In his 2006 "Linked Data" note, Tim Berners-Lee outlined four principles of linked data, paraphrased along the following lines:
Uniform Resource Identifiers (URIs) should be used to name and identify individual things.
HTTP URIs should be used to allow these things to be looked up, interpreted, and subsequently "dereferenced".
Useful information about what a name identifies should be provided through open standards such as RDF, SPARQL, etc.
When publishing data on the Web, other things should be referred to using their HTTP URI-based names.

Tim Berners-Lee later restated these principles at a 2009 TED conference, again paraphrased along the following:
All conceptual things should have a name starting with HTTP.
Looking up an HTTP name should return useful data about the thing in question in a standard format.
Anything else that that same thing has a relationship with through its data should also be given a name beginning with HTTP.

Components
 URIs
 HTTP
 Structured data using controlled vocabulary terms and dataset definitions expressed in Resource Description Framework serialization formats such as RDFa, RDF/XML, N3, Turtle, or JSON-LD
 Linked Data Platform

Linked open data
Linked open data are linked data that are open data. Tim Berners-Lee gives the clearest definition of linked open data in differentiation with linked data.
 

Large linked open data sets include DBpedia and Wikidata.

5-star linked open data 

Tim Berners-Lee has suggested a 5-star scheme for grading the quality of open data on the web, for which the highest ranking is Linked Open Data:

 1 star: data is openly available in some format.
 2 stars: data is available in a structured format, such as Microsoft Excel file format (.xls).
 3 stars: data is available in a non-proprietary structured format, such as Comma-separated values (.csv).
 4 stars: data follows W3C standards, like using RDF and employing URIs. 
 5 stars: all of the other, plus links to other Linked Open Data sources.

History
The term "linked open data" has been in use since at least February 2007, when the "Linking Open Data" mailing list was created. The mailing list was initially hosted by the SIMILE project at the Massachusetts Institute of Technology.

Linking Open Data community project

The goal of the W3C Semantic Web Education and Outreach group's Linking Open Data community project is to extend the Web with a data commons by publishing various open datasets as RDF on the Web and by setting RDF links between data items from different data sources. In October 2007, datasets consisted of over two billion RDF triples, which were interlinked by over two million RDF links.  By September 2011 this had grown to 31 billion RDF triples, interlinked by around 504 million RDF links.  A detailed statistical breakdown was published in 2014.

European Union projects
There are a number of European Union projects involving linked data. These include the linked open data around the clock (LATC) project, the PlanetData project, the DaPaaS (Data-and-Platform-as-a-Service) project, and  the Linked Open Data 2 (LOD2) project. Data linking is one of the main goals of the EU Open Data Portal, which makes available thousands of datasets for anyone to reuse and link.

Ontologies
Ontologies are formal descriptions of data structures. Some of the better known ontologies are:

 FOAF – an ontology describing persons, their properties and relationships
 UMBEL – a lightweight reference structure of  subject concept classes and their relationships derived from OpenCyc, which can act as binding classes to external data; also has links to 1.5 million named entities from DBpedia and YAGO

Datasets
 DBpedia – a dataset containing extracted data from Wikipedia; it contains about 3.4 million concepts described by 1 billion triples, including abstracts in 11 different languages
 GeoNames – provides RDF descriptions of more than  geographical features worldwide.
 Wikidata – a collaboratively-created linked dataset that acts as central storage for the structured data of its Wikimedia Foundation sibling projects
 Global Research Identifier Database (GRID) – an international database of  institutions engaged in academic research, with  relationships, models two types of relationships: a parent-child relationship that defines a subordinate association, and a related relationship that describes other associations

Dataset instance and class relationships
Clickable diagrams that show the individual datasets and their relationships within the DBpedia-spawned LOD cloud (as shown by the figures to the right) are available.

See also
 American Art Collaborative - consortium of US art museums committed to establishing a critical mass of linked open data on American art
 Authority control – about controlled headings in library catalogs
 Citation analysis – for citations between scholarly articles
 Hyperdata
 Network model – an older type of database management system
 Schema.org
 VoID – Vocabulary of Interlinked Datasets
 Web Ontology Language

References

Further reading

 Ahmet Soylu, Felix Mödritscher, and Patrick De Causmaecker. 2012. “Ubiquitous Web Navigation through Harvesting Embedded Semantic Data: A Mobile Scenario.” Integrated Computer-Aided Engineering 19 (1): 93–109.
 Linked Data: Evolving the Web into a Global Data Space (2011) by Tom Heath and Christian Bizer, Synthesis Lectures on the Semantic Web: Theory and Technology, Morgan & Claypool 
 How to Publish Linked Data on the Web, by Chris Bizer, Richard Cyganiak and Tom Heath, Linked Data Tutorial at Freie Universität Berlin, Germany, 27 July 2007.
 The Web Turns 20: Linked Data Gives People Power, part 1 of 4, by Mark Fischetti, Scientific American 2010 October 23
 Linked Data Is Merely More Data – Prateek Jain, Pascal Hitzler, Peter Z. Yeh, Kunal Verma, and Amit P. Sheth. In: Dan Brickley, Vinay K. Chaudhri, Harry Halpin, and Deborah McGuinness: Linked Data Meets Artificial Intelligence. Technical Report SS-10-07, AAAI Press, Menlo Park, California, 2010, pp. 82–86.
 Moving beyond sameAs with PLATO: Partonomy detection for Linked Data – Prateek Jain, Pascal Hitzler, Kunal Verma, Peter Z. Yeh, Amit Sheth. In:  Proceedings of the 23rd ACM Hypertext and Social Media conference (HT 2012), Milwaukee, WI, USA, June 25–28, 2012.
 Freitas, André, Edward Curry, João Gabriel Oliveira, and Sean O’Riain. 2012. “Querying Heterogeneous Datasets on the Linked Data Web: Challenges, Approaches, and Trends.” IEEE Internet Computing 16 (1): 24–33.
 Interlinking Open Data on the Web – Chris Bizer, Tom Heath, Danny Ayers, Yves Raimond. In Proceedings Poster Track, ESWC2007, Innsbruck, Austria
 Ontology Alignment for Linked Open Data – Prateek Jain, Pascal Hitzler, Amit Sheth, Kunal Verma, Peter Z. Yeh. In proceedings of the 9th International Semantic Web Conference, ISWC 2010, Shanghai, China
 Linked open drug data for pharmaceutical research and development - J Cheminform. 2011; 3: 19. Samwald, Jentzsch, Bouton, Kallesøe, Willighagen, Hajagos, Marshall, Prud'hommeaux, Hassenzadeh, Pichler, and Stephens (May 2011)
 Interview with Sören Auer, head of the LOD2 project about the continuation of LOD2 in 2011, June 2011
 Linked Open Data: The Essentials - Florian Bauer and Martin Kaltenböck (January 2012)
 The Flap of a Butterfly Wing - semanticweb.com Richard Wallis (February 2012)

External links

 LinkedData at the W3C Wiki
 LinkedData.org
 OpenLink Software white papers
 Data from Northwind SQL schema as linked data, use case demo
 Linked data for the discipline of numismatics, use case demo
 Interactive LOD demo
 OptiqueVQS -- an ontology-based visual query formulation tool

Computer-related introductions in 2007
Cloud standards
Data management
Distributed computing architecture
Hypermedia
Internet terminology
Open data
World Wide Web
Semantic Web