Schema.org
Definition
Schema.org is a collaborative vocabulary of types and properties for describing things on the web in a machine-readable way.
Full definition
Types such as Organization, Product, FAQPage, and BreadcrumbList define standard fields (name, url, offers, and so on). Publishers express those types via JSON-LD, Microdata, or RDFa.
Search engines use Schema.org extensively for rich results and understanding. Not every type unlocks a rich result for every site; validity and eligibility differ. See Structured Data.