The Semantic Web was first coined by the inventor of WWW, Tim Berners-Lee, approximately fifteen years ago . The Semantic Web is envisioned to transform the content of the Web into machine-understandable format rather than human-readable. It allows machines to process and concludes the consequences from the original sources. The web evolves from information providers into knowledge.
However, the Semantic Web is not the separation of current Web, it is an extension of current one, in which information is given well-defined meaning, better, enabling computers and people to work in cooperation . In other words, the current Web is the Web for people within HTTP, XML, HTML, and URI technologies, whereas the Semantic Web is the Web for machines with the resources are more structured. Since it transforms into well-structured format, the semantic web is somehow to be called as web of data and linked data.
To provide a well-structured of web of data, the data should be described into more abstract format which is ontology. The ontology is a pillar of semantic web technologies. For example, the ontology below represent Student, Module, and University.
Fig 1. Example of Ontology
The ontology is only a conceptual modelling to represent real world knowledge. It needs to be presented into more well-form serialization such as RDF Statement which can be read by a machine. Every statement should be described as a triple which consists of subject, predicate, and object and to be linked using URI (Universal Resource Identification). The idea of those URI brings the web into a linked data form which every single data is connected within URI.
Fig 2. Example of Graph Statement
The graph above is an instance for the ontology (Fig 1.) which states that instance IisAfriyanti studies_at UniversityOfSouthampton and IisAfriyanti hasModule SemanticWeb which can be represented as RDF below:
The graph above can be consumed using SPARQL, a SQL-Like query. For example to retrieve all student with its modules by querying:
PREFIX unisocial: <http://unisocial.com/opendata#>
SELECT ?student ?module
?student unisocial:hasModule ?module
The idea of semantic web technology above leads the concept of social semantic web which is put social context into semantic web technologies. This concept rises since the problem within social networks; the interoperability on the social networks websites . A single user might has more than one account of social network and he or she needs to put their personal information into different social network they have which are not interlinked.
The best example of ontology to describe a person and his interaction with his friends is probably FOAF vocabulary (Friend Of A Friend). This ontology contains many classes such as Person, Organization, which many properties available such as name, title, mbox, homepage, to describe a person and property knows to link between person.
 Tim Berners-Lee, James Hendler, and Ora Lasilla. The Semantic Web. Scientific American, 284(5):34-43, 2001.
 Mäkeläinen, Sami I. Social Semantic Web. Paper for ”Tiedonhallinta Semanttisessa Webissä”-seminar, University of Helsinki, 2005.