SPARQL Query Language for RDF

DBpedia is a community effort to extract structured information from Wikipedia and to make this information available on the Web. DBpedia allows you to ask sophisticated queries against Wikipedia and to link other datasets on the Web to Wikipedia data.
[source: www.DBPedia.org]
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The W3C just gave SPARQL the stamp of approval. SPARQL is a query language for the Semantic Web, and differs from othe query languages in that is usable across different data sources. There are allready 14 implementations of the spec, which is a lot. Most of them are free software. There are also billions of relations out there that are queryable, thanks to the Linking Open Data project. The structured data of Wikipedia are now queryable at DBpedia. Also, have a look at Ivan Herman's presentations.

Lets have an example: You could do this on dbpedia.org (with the standard prefixes you find there) and it will return computer scientists born before 1945: SELECT ?name ?birth ?death ?person WHERE { ?person skos:subject . ?person dbpedia2:birth ?birth . ?person foaf:name ?name . OPTIONAL { ?person dbpedia2:death ?death } FILTER (?birth < "1945-01-01"^^xsd:date) . } ORDER BY ?name
[source: Slashdot]
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SPARQL Query Language for RDF
ABSTRACT
RDF is a directed, labeled graph data format for representing information in the Web. This specification defines the syntax and semantics of the SPARQL query language for RDF. SPARQL can be used to express queries across diverse data sources, whether the data is stored natively as RDF or viewed as RDF via middleware. SPARQL contains capabilities for querying required and optional graph patterns along with their conjunctions and disjunctions. SPARQL also supports extensible value testing and constraining queries by source RDF graph. The results of SPARQL queries can be results sets or RDF graphs.