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The idea of the Semantic Web was conceived by Tim Berners-Lee, the founder of the World Wide Web cite{berners}. He envisions that in the future the vast amount of information on the Web will be linked with machine readable meta-data, enabling computers to use the contents for automation, integration and re-use cite{semantic}. Therefore, the Semantic Web is imagined as an extension of the Web, in which information is given well-defined meaning. This way machines would be able to semantically interpret information on the Web and respond to complex human requests subject to their meaning. The Semantic Web has been promoted as a long-term project by the World Wide Web Consortium (W3C), the organization that is responsible for setting technical standards on the Web cite{w3c}. Advancements in the fields of knowledge representation, automated reasoning and AI have been used to support the move towards the Semantic Web cite{giri}. Ontologies are defined as “explicit specifications of shared conceptualizations” cite{ontology}. A conceptualization can be described as an abstract and simplified view of the real world cite{studer}. This way ontologies can be seen as tools for modeling an abstract view of the real world and enabling machines to analyze the contextual semantics of documents. Therefore, the Semantic Web heavily relies on formal ontologies for organizing underlying data for the purpose of machine understanding.   paragraph{ extbf{The Semantic Web stack}} The Semantic Web contains resources that do not only correspond to media objects as the current Web does, but also to real-world objects such as people, places, organizations and events. According to Tim Berners Lee, the Semantic Web will consist of different types of relations amongst the resources: “Computers will find the meaning of semantic data by following hyperlinks to definitions of key terms and rules for reasoning about them logically” cite{berners}.Figure
ef{stack} illustrates the architecture of the Semantic Web. It shows an hierarchy of languages and technologies needed to realize the full vision of the Semantic Web cite{w3c2}. Each layer exploits and uses capabilities of the layers below. The bottom layers up to XML contain technologies that are well known from hypertext web and that without further change provide basis for the Semantic Web. The middle layers contain technologies standardized and accepted by W3C for building Semantic Web applications. As illustrated, the Semantic Web builds on the following technologies and languages cite{w3c} cite{berners2}: