What is Semantic Web

Semantic Web : The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries. It is a collaborative effort led by W3C with participation from a large number of researchers and industrial partners. It is based on the Resource Description Framework (RDF), which integrates a variety of applications using XML for syntax and URIs for naming.

"The Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabling computers and people to work in cooperation."

The Semantic Web is comprised of the standards and tools of XML, XML Schema, RDF, RDF Schema and OWL. The OWL Web Ontology Language Overview describes the function and relationship of each of these components of the Semantic Web:
XML provides a surface syntax for structured documents, but imposes no semantic constraints on the meaning of these documents.
XML Schema is a language for restricting the structure of XML documents.
RDF is a simple data model for referring to objects ("resources") and how they are related. An RDF-based model can be represented in XML syntax.

RDF Schema is a vocabulary for describing properties and classes of RDF resources, with a semantics for generalization-hierarchies of such properties and classes.
OWL adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. "exactly one"), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.

Check out http://en.wikipedia.org/wiki/Semantic_web