elaborate on difference between sparql and rdf4j

doc/slides/slides.tex

@@ -185,11 +185,17 @@
   \begin{itemize}
     \item Based on our implementation of Collector, Importer and
       Endpoint we implemented some applications and queries.
-    \item Our endpoint supports both SPARQL~\cite{sparql} and
+    \item Our Endpoint supports both SPARQL~\cite{sparql} and
       RDF4J~\cite{rdf4j}. Both have their advantages.
+      \begin{itemize}
+        \item SPARQL is an universal query language for RDF~triplet
+            data.
+          \item RDF4J is a JVM-based library that builds on top of
+            {SPARQL}. It does not do more, but it can be more
+            convenient.
+      \end{itemize}
     \item While the applications themselves are not terribly
-      interesting, they lead us to some interesting questions. This
-      will be the focus of this discussion.
+      interesting, they lead us to some interesting questions.
   \end{itemize}
 \end{frame}