Structured data is standardized code you add to a page to describe its content in a way machines can parse, using a shared vocabulary such as Schema.org. It helps search engines identify the entities on a page — a product, a recipe, an event — and can make the page eligible for enhanced listings in search.
Two things get confused here: the vocabulary and the format. Schema.org is the vocabulary — the shared list of types and properties like Product, Review, or FAQPage; see schema markup for that side of it. The format is how you write the vocabulary into a page: JSON-LD, Microdata, or RDFa. Google recommends JSON-LD, a block of script you drop into the head or body without touching the visible HTML, which makes it the easiest to add and maintain.
Marking up a page makes it eligible for rich results, though eligibility is never a guarantee. A recipe page that adds Recipe schema with a rating, cook time, and calories can appear with a star rating, "45 min," and a thumbnail — a listing that visibly outsizes plain blue links and lifts click-through. Increasingly, structured data also helps AI systems read a page accurately, feeding cleaner facts into AI Overviews and entity understanding. Implementing and validating markup is a standard part of technical SEO.