Embedding data / metadata in HTML opens traditional Web 2.0 development to Web 3.0 (aka semantic) technologies.
Well-defined terminologies in form of vocabularies (such as Dublin Core) or full-featured Ontologies (which can be constructed using meta languages such as RDF and OWL) provide the basis for a logically rich interoperability layer in the visible and unvisible Web.
Microformats, RDFa and microdata all enable consumers to extract data from HTML pages. This data may be embedded within enhanced search engine results, exposed to users through browser extensions, aggregated across websites or used by scripts running within those HTML pages.
HTML microdata [MICRODATA] is an extension to HTML used to embed machine-readable data into HTML documents. Whereas the microdata specification describes a means of markup, the output format is JSON. This specification describes processing rules that may be used to extract RDF [RDF-CONCEPTS] from an HTML document containing microdata.
Source: W3C’s news article