Schema.org is a collaboration from several search engines, defining logic data model and vocabulary for structured data based on entity relationships model. It has proposed in the model the set of entities, their object and data properties.
Schema.org is complex data model proposed by Google, Yahoo, others for search engine optimization task. The sets of entities were described by object-properties which can be described by an instance of other entity. As result, proposed data model describes so deeply the complex graph-oriented relationships among entities and their properties what is very useful for working with ontology and knowledge-driven intellectual systems.
Schema.org is a community-created vocabulary (sponsored by Google, Microsoft, Yahoo and Yandex) for describing entities on webpages, in emails etc. It is commonly used with the json-ld, microdata, or rdfa syntax and it is considering a markup support for search engine too.
What consumers (like search engines) may do with Schema.org markup:
Official website: http://schema.org/
Wikipedia page: https://en.wikipedia.org/wiki/Schema.org
W3C Community Group: https://www.w3.org/community/schemaorg/
Mailing list: http://lists.w3.org/Archives/Public/public-schemaorg