Schema.org
Schema.org is the shared vocabulary created by the major search engines to describe page content: organizations, articles, products, events, reviews. It is implemented preferably in JSON-LD, embedded in the page code.
In practice
The vocabulary runs to hundreds of types; a handful actually matter: Organization, LocalBusiness, Article, Product, FAQPage, BreadcrumbList. Types combine into a graph — the article references its publisher, the publisher references its logo and address. Markup errors are checked with official testing tools before launch, never after.