“ I have a dream for the Web [in which computers] become capable of analyzing all the data on the Web – the content, links, and transactions between people and computers. A ‘Semantic Web’, which should make this possible, has yet to emerge, but when it does, the day-to-day mechanisms of trade, bureaucracy and our daily lives will be handled by machines talking to machines. The ‘intelligent agents’ people have touted for ages will finally materialize. ”
—Tim Berners-Lee, 1999
Imagine the day, when you get a single customized answer to a certain question (e.g. What is the cheapest car in Europe?), when you ask the internet for a suitable clinic by entering the symptoms and your location and the internet responds with the predicted disease, clinic name and reservation times, and when every clinic and hospital all over the world share their data forming a large set of data through the internet, by using some software agents, imagine what can come out of billions of records. This is called the Semantic Web or Web 3.0.
According to Wikipedia, Semantic web is an extension of the World Wide Web in which web content can be expressed not only in natural language, but also in a format that can be read and used by software agents. In other words, Data from different sources should be represented in a common format that is readable by machines to easily integrate and combine them, also using natural languages to describe how data relates to real world objects. Today's WWW is said to be web of documents, that is data is controlled by applications and each application keeps it to itself, those applications interchange documents. Semantic Web is said to be web of data, because using common formats to represent data eases integrating it, and using natural language to relate it data to real world, allows data to be shared and reused across applications, enterprise and community boundaries. The Semantic Web is based on Resource Description Framework (RDF).
RDF is a family of World Wide Web Consortium (W3C) specifications originally designed as a metadata model but which has come to be used as a general method of modeling information, through a variety of syntax formats and notations. These syntax formats includes Notation 3 (N3), RDF/XML and N-Triples, while notations include RDF Schema (RDFS) and the Web Ontology Language (OWL).
Watch this video in which Tim Berners-Lee explains what is the Semantic Web.