In general, semantics is the study of meaning (The word "semantic" comes from the Greek semantikos, or "significant meaning," derived from sema, or "sign." )
Semantic Web technologies help separate meanings from data, document content, or application code, using technologies based on open standards. If a computer understands the semantics of a document, it doesn't just interpret the series of characters that make up that document: it understands the document's meaning.
The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries. You can think of the Semantic Web as an efficient way to represent data on the World Wide Web, or as a database that is globally linked, in a manner understandable by machines, to the content of documents on the Web. Semantic technologies represent meaning using ontologies and provide reasoning through the relationships, rules, logic, and conditions represented in those ontologies.
To represent the Semantic Web, you'll use the following technologies:
- A global naming scheme (URIs)
- A standard syntax for describing data (RDF)
- A standard means of describing the properties of that data (RDF Schema)
- A standard means of describing relationships between data items (ontologies defined with the OWL Web Ontology Language)
Each of these concepts is well explained in my article - http://www.ibm.com/developerworks/web/library/wa-semweb/