Semantic Web is an extension of the traditional web that links objects or concepts through semantic relationships.
Search engines retrieve their request scanning the information and looking for keywords. However, it doesn´t take in consideration the context or the meaning of the words in each particular situation. That is the goal of the semantic web.
For that, the semantic web use a common format or language (RDF), a specific way to get access to that data and find those relationships (SPARQL), and a set of vocabularies or ontologies that define concepts and relationships used to describe specific objects (OWL). With all those elements the system look at the possible relationships between terms and meanings and suggest a final result (inference).