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Conventional information retrieval techniques are document-centric search paradigm. Users are required to investigate an document or lists of documents in order to obtain an answer.
Semantic serach is an object, entity or knowledge centric approach based on the semantic web technologies.

It aims to find precise and abundant information of the objects under consideration and their related objects. Semantic search technology enables accurate retrieval of information via concept/meaning match. It is very effective, and perhaps the only method, in application to credible and dynamic content. Because most of the credible and dynamic content are statistically flat (infertile) for popularity algorithms to work effectively beyond common queries.

hakia (http://www.hakia.com/)  is a general purpose “semantic” search engine, dedicated to quality search experience.

For more information, you may refer to An Overview of Semantic Search Engines.

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The Resource Description Framework (RDF) is a W3C Recommendation for the formulation of metadata on the World Wide Web. RDF Schema (RDFS) extends this standard with the means to specify domain vocabulary and object structures. These techniques will enable the enrichment of the Web with machine-processable semantics, thus giving rise to what has been dubbed the Semantic Web.

  

The basic building block in RDF is an subject-predicate-object triple, commonly written as P(S;O). That is, a subject S has an predicate (or property) P with value O. Another way to think of this relationship is as a labeled edge between two nodes: [S] –P–>[O].

  

The RDF data model provides no mechanisms for declaring vocabulary that is to be used. RDF Schema is a mechanism that lets developers define a particular vocabulary for RDF data (such as the predicate hasWritten) and specify the kinds of objects to which predicates can be applied (such as the class Writer). RDFS does this by pre-specifying some terminology, such as Class, subClassOf and Property, which can then be used in application-specific schemata.

 

RDFa was first proposed by Mark Birbeck in the form of a W3C note entitled XHTML and RDF, which was then presented to the Semantic Web Interest Group at the W3C’s 2004 Technical Plenary. RDF/A is a set of attributes used to embed RDF in XHTML. 

 

For more information, watch this video : http://www.youtube.com/watch?v=ldl0m-5zLz4

Moreover, have a look to the  presentation : RDFa – Bridging the Web of Documents and the Web of Data” that will be presented in ISWC2008.