what is semantic web?

"If HTML and the Web made all the online documents look like one huge book, RDF, schema, and inference languages will make all the data in the world look like one huge database"
Tim Berners-Lee, Weaving the Web, 1999
The major Building Blocks for Semantic Web

1-RDF
2-RDFS
3-OWL
4-Ontologies

These are all together deliver the functionality of semantic web.

The Semantic Web is not about links between web pages.
The Semantic Web describes the relationships between things (like A is a part of B and Y is a member of Z) and the properties of things (like size, weight, age, and price).

RDF (The Resource Description Framework)

RDF (Resource Description Framework) is a markup language for describing information and resources on the web.
Putting information into RDF files, makes it possible for computer programs ("web spiders") to search, discover, pick up, collect, analyze and process information from the web.
The Semantic Web uses RDF to describe web resources.

RDFS (RDF Schema)

RDF describes resources with classes, properties, and values.
In addition, RDF also need a way to define application-specific classes and properties. Application-specific classes and properties must be defined using extensions to RDF
One such extension is RDF Schema.

OWL (Web Ontology Language)

OWL stands for Web Ontology Language
OWL is built on top of RDF
OWL is for processing information on the web
OWL was designed to be interpreted by computers
OWL was not designed for being read by people
OWL is written in XML
OWL has three sublanguages
OWL is a web standard

Ontology

Ontology is about the exact description of things and their relationships.
For the web, ontology is about the exact description of web information and relationships between web information.