SPARQL is the de facto standard query language for RDF (it’s now in last call) and many think that it will play a major role in hastening the adoption of the W3C’s vision of a Web of Data. I think Ora Lassila and Jim Hender said it very well in their recent Internet computing article, Embracing Web 3.0:
“SPARQL’s emergence as the standard query language for RDF lets many data stores expose themselves as SPARQL endpoints, thus enabling flexible data exchange among systems. It is leading the way toward Web applications that exhibit a kind of “fractal” structure, with patterns in which one component uses another as a data source (via SPARQL, for example) and acts as a data source to yet another component. Such architectures open up new possibilities for the original vision of Web services and loosely coupled distributed systems.”
OpenLink Software has a great interactive SPARQL demo that is very helpful in exploring both the SPARQL language and protocol. You can try out any of the RDF Data Access Working Group (DAWG) test case queries or enter your own ad hoc ones, on their data or yours. You an also easily see the actual request and response that go over the wire. I think this is a great aid to deepening your understanding of SPARQL.