The server-side part of OpenRefine is implemented in Java as one single servlet which is executed by the Jetty web server + servlet container. The use of Java strikes a balance between performance and portability across operating system (there is very little OS-specific code and has mostly to do with starting the application).
OpenRefine has no database using its own in-memory data-store that is built up-front to be optimized for the operations required by faceted browsing and infinite undo.
The functional extensibility of OpenRefine is provided by the SIMILE Butterfly modular web application framework.
Several projects provide the functionality to read and write custom format files (POI, opencsv, JENA, marc4j).
String clustering is provided by the SIMILE Vicino project.
OAuth functionality is provided by the Signpost project.