From W3C Wiki

The Virtuoso Sponger ("Sponger") is an RDF middleware layer (built into Virtuoso's SPARQL Processor, Web Content Crawler, and WebDAV RDF Sink Folder) that produces RDF instance data from non-RDF Data Sources (i.e., an RDFizer).

The Sponger is comprised of Cartridges that host Ontology Mappers and Metadata Extractors. The Ontology Mappers cover a wide range of shared ontologies, while the Metadata Extractors cover a wide range of (X)HTML, XML, XML Web Services, and Binary Files.

The production of RDF from RDF follows the sequence below:

  1. Metadata Extractor extracts metadata from data source (e.g., a web resource)
  2. Extracted Metadata is processed using one or more Ontology Mappers to produce RDF instance Data

Additional Information:

  1. Sponger Technical White Paper (PDF & HTML)