Schema markup is structured-data code — most often written in JSON-LD — that you add to a page to spell out what its content means, so search engines can show enhanced listings such as star ratings, FAQ drop-downs, prices, and event dates. It draws on the shared vocabulary published at Schema.org.
A crawler reads your visible text easily, but it still has to infer relationships — is "4.8" a price, a rating, or a version number? Schema removes that ambiguity. You place a small JSON-LD script in the page describing an entity and its properties (for a product: name, brand, price, currency, review count, average rating). Google validates it, and when the data matches what's actually on the page, the page becomes eligible for a rich result. Schema markup is one specific vocabulary within the broader idea of structured data; the terms are often used interchangeably, but structured data is the concept and Schema.org is the dictionary.
Consider a small bakery that adds Product and AggregateRating markup to 40 cake pages. Within three weeks Google starts rendering a 4.8-star badge beside those listings; click-through climbs from 2.1% to 3.6% while average position barely moves. The rankings did not change — the result simply became more clickable. Marking a page up does not guarantee a rich snippet, though; eligibility is always Google's call.
Not as a direct ranking factor. It improves how your result looks and can lift click-through, which supports performance indirectly. Implementation is usually handled as part of technical SEO.
JSON-LD. Google recommends it over Microdata or RDFa because it sits in one block, is easy to maintain, and doesn't tangle with your visible HTML.