Wikipedia
Schema.org is an initiative launched on 2 June 2011 by Bing, Google and Yahoo! (the operators of the then world's largest search engines) to “create and support a common set of schemas for structured data markup on web pages.” In November 2011 Yandex (whose search engine is the largest one in Russia) joined the initiative. They propose using the schema.org vocabulary along with the Microdata, RDFa, or JSON-LD formats to mark up website content with metadata about itself. Such markup can be recognized by search engine spiders and other parsers, thus gaining access to the meaning of the sites (see Semantic Web). The initiative also describes an extension mechanism for adding additional properties. Public discussion of the initiative largely takes place on the W3C public vocabularies mailing list.
Much of the vocabulary on schema.org was inspired by earlier formats such as Microformats, FOAF, GoodRelations and OpenCyc. Microformats, with its most dominant representative hCard, continue to be published widely in the Web, where the deployment of schema.org has strongly increased between 2012 and end 2014.
RDF applications can use Microdata2RDF service. getSchema is a community wiki maintaining a set of markup examples.
A mapping from the terms defined in Schema.org to RDF (expressed in RDF Schema) is available.
To test the validity of the data marked up with the schemas and Microdata, such validators as the Google Structured Data Testing Tool, Yandex Microformat validator and Bing Markup Validator can be used.
Some Schema markups such as Organization and Person are used to influence Google's Knowledge Graph results.