Structured data is markup that describes a page's content in a standardised, machine-readable format. The dominant vocabulary is Schema Markup (Schema.org), maintained jointly by Google, Bing, Yandex, and Yahoo. The dominant serialisation format is JSON-LD, embedded in the page's head or body as a JSON object inside a <script type="application/ld+json"> tag.
The terms "structured data" and "schema markup" are often used interchangeably. The technical distinction: structured data is the broader category (any machine-readable description of content), while Schema.org is one specific vocabulary, and JSON-LD is one specific format for expressing it. For practical SEO purposes, "structured data," "schema," and "JSON-LD" usually all mean the same thing.
Why structured data matters: it enables rich SERP features that improve visibility and click-through. Product schema makes star ratings and prices appear in search results. Article schema surfaces publication dates and author bylines. Recipe schema produces image carousels. BreadcrumbList replaces URL displays with breadcrumb trails. DefinedTerm schema (used on this glossary) makes glossary entries eligible for definition-style rich results.
Structured data also feeds the Knowledge Graph — Google's internal entity database. Organization, Person, and Place schema help Google connect a website to a specific real-world entity, which can produce knowledge panels in branded search results and improve disambiguation for ambiguous queries.
Implementation discipline is critical. The structured data must accurately reflect what is on the page. Marking up prices that do not exist, ratings without a real review system, or events with fabricated dates is against Google's structured data guidelines and can result in manual actions revoking rich result eligibility — sometimes site-wide.
Validate before deploying with Google's Rich Results Test (the modern replacement for the Structured Data Testing Tool). Monitor Search Console's Enhancements reports for ongoing validation errors. Schema.org's vocabulary is broad, but only a subset is supported for visible Google rich results — check Google's Search Gallery documentation for the current supported types.
For most modern sites, the baseline set is Organization, WebSite, BreadcrumbList, and a content-type-specific schema per template. Add specialised types only where they accurately describe the content.
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