SPARQL is a W3C standard recommendation to query RDF graphs. It comes with a notation similar to the relational database query language SQL, but focuses on triple matching. Please check the SPARQL specification online for a precise language description:
http://www.w3.org/TR/rdf-sparql-query/
SPARQL can also be used for tasks such as ontology mapping and inferencing. TopBraid Composer provides comprehensive SPARQL support using SPIN.