Structured data is metadata that describes a page's entities (article, product, recipe, event, FAQ) using a vocabulary search engines understand. The dominant vocabulary is schema.org, jointly maintained by Google, Microsoft, Yahoo, and Yandex. The dominant format is JSON-LD embedded in <script type="application/ld+json"> blocks.
Structured data unlocks "rich results" in Google: star ratings, FAQ accordions, recipe cards with cook time, event date pickers, breadcrumb trails. Each rich result type has specific required + recommended properties documented at developers.google.com/search/docs/appearance/structured-data.
Common types worth implementing: BreadcrumbList (every internal page), FAQPage (any page with question-shaped sections), Article / TechArticle (editorial content), Product + AggregateRating (e-commerce), Organization (homepage). Validate with Google's Rich Results Test before shipping.