Schema.org
A shared vocabulary of structured-data types that lets sites describe their content in a way machines can reliably read.
What is Schema.org?
Schema.org is a shared vocabulary (a set of agreed-upon labels like Product, Review, or Event) that website owners use to mark up their content so computers, not just humans, can reliably tell what a page is about. Without it, a search engine has to guess from surrounding text that a number on a page is a "price"; with schema.org markup, the page explicitly labels it as one.
Why it matters
Schema.org launched in 2011 as a joint initiative of Google, Microsoft (Bing), Yahoo, and later Yandex, specifically to converge on one vocabulary instead of each search engine promoting its own competing markup format. The vocabulary is organized as a hierarchy of types (Product is a subtype of the more general Thing), each carrying a defined set of properties, and it's deliberately format-agnostic. The same schema.org types can be expressed in JSON-LD, Microdata, or RDFa. The project is maintained through an open community process with public source repositories and working groups, and now covers thousands of types spanning far beyond search, including use in knowledge graphs and, increasingly, as a structured input format AI systems parse when summarizing web content.