The Semantic Web refers to the W3C’s vision of a Web of linked data complementing the well known Web of documents. The goal of the Web of linked data is to enable computers to do more useful work and to develop systems that can support trusted interactions over the network.
The Semantic Web Standards supporting the Web of linked data include RDF, SPARQL, OWL, and SKOS. These standards build on a core Web Architecture and enable people to create data stores on the Web, build vocabularies (ontologies), and write rules for handling data. The common framework provided by the Semantic Web Standards allows data to be shared and reused across application, enterprise, and community boundaries.
The Schemapedia Index of Schemas is a good place to find an ontology describing a particular domain.