Short definition
JSON-LD (JSON for Linking Data) is the format Google recommends for embedding schema markup, written as a JSON object in a <script type='application/ld+json'> tag.
In depth
JSON-LD is preferred over the older Microdata and RDFa formats because it lives in a single block and is decoupled from HTML structure, making it easier to maintain and audit. A page can carry multiple JSON-LD blocks or one combined @graph that links related entities by @id. Validation is done via Google's Rich Results Test and the Schema.org validator.
Example
A site combines Organization, LocalBusiness, WebSite, and Service into a single @graph block, deduped by @id. Rich Results Test passes with no warnings.