Semantic Web
The Semantic Web is a concept originated by Tim Berners-Lee and a set of technologies and practices developed out of this vision. Simply put, the Semantic Web is a Web whereby collections of assertions about the state of the world are linked together in much the same way as web pages are. What differentiates the Semantic Web from existing data structures is the use of URIs to uniquely identify things, and relationships between things. The sort of problem scenario that Semantic Web technologies try to solve are those involving multiple disparate source of data - for instance, hooking together train timetables and class timetables, so a student can automatically plan their travel itinerary without having to manually match the data together.
The W3C have put forward a variety of standards built on top of the Resource Description Framework, a formal semantic model for representing things and the relationships between them.