Maintaining Activity: Sematic Web Activity
RDFa is a specification for attributes that can be used with XHTML to express structured machine-readable data within an XHTML document. Like microformats, RDFa allows parts of the HTML content to be used both for rendering (to the human reader) and be part of the structured (machine-readable) data. Unlike microformats, RDFa specifies only a syntax for including structured data and relies on independently-developed vocabularies (also called taxonomies or ontologies) to provide the specific terms for given data items. RDFa permits these vocabularies to be freely mixed within a single XHTML document.
Additional information may be found in the RDFa Syntax specification, the RDFa Primer, and the RDFa community wiki.