Schema.org
Schema.org is an open vocabulary of several hundred types and properties, maintained jointly by Google, Microsoft, Yahoo and Yandex, that describes web content such as organisations, products, articles or terms in a uniform, machine-readable way.
In practice
Schema.org itself is not a file format, only the vocabulary; technically it is usually implemented through JSON-LD. Using the appropriate types consistently makes content clearly identifiable for search engines and AI systems, instead of merely open to textual interpretation.