Schema.org is a community-created vocabulary (sponsored by Google, Microsoft, Yahoo and Yandex) for describing entities on webpages, in emails etc. It can be used with different syntaxes (most commonly JSON-LD, Microdata, and RDFa).
What consumers (like search engines) may do with Schema.org markup:
Official website: http://schema.org/
W3C Community Group: https://www.w3.org/community/schemaorg/
Mailing list: http://lists.w3.org/Archives/Public/public-schemaorg