Structured Data
Structured data is machine-readable information added to a web page, usually following the shared vocabulary at schema.org, that describes what the page is about (a product, a recipe, a review) in a fixed format so search engines and AI systems can read it without guessing.
The term is often used interchangeably with schema markup, but they are not the same thing. Structured data is the broad idea of describing a page in a defined, machine-readable shape; schema markup is the specific implementation using the schema.org vocabulary, which is by far the most common one on the open web. In practice, when people say structured data for SEO they almost always mean schema.org types expressed in a supported format.
There are three formats Google supports: JSON-LD, Microdata, and RDFa. JSON-LD is the recommended one because it sits in a script block separate from the visible HTML, so it is easier to add, read, and maintain. The same data can power a rich result in Google (star ratings, prices, FAQ accordions) and give AI assistants a clean, unambiguous source to quote, rather than parsing it out of page copy.
The honest caveat: structured data does not guarantee a rich result or a citation, and it must accurately reflect what is visible on the page. Marking up ratings or reviews that a shopper cannot actually see is against Google policy and can trigger a manual action. The data has to be true and corroborated, not decorative.