JSON-LD
JSON-LD (JavaScript Object Notation for Linked Data) is the Google-preferred format for structured data: a script block of linked-data JSON, placed in a page, that describes the entity on that page to search engines without touching the visible HTML.
JSON-LD lives in a script tag with the type application/ld+json, usually in the head or body of the page. Because it sits apart from the rendered markup, it does not interleave attributes through your HTML the way microdata and RDFa do, which is why Google recommends it and why it is the easiest format to add, audit, and remove without disturbing the layout.
A single block can describe a product, an article, an organisation, a breadcrumb trail, or aggregated review ratings, and several blocks can coexist on one page. The format is just JSON, so it is straightforward for a template to generate and for a validator to read, and it can be injected after page load by tag managers when you cannot edit the template directly.
The honest caveat is that JSON-LD must match what a visitor actually sees. Google treats structured data that claims ratings, prices, or content not present on the page as a guidelines violation, and it can suppress rich results or issue a manual action. Keep the JSON synchronised with visible content, and treat it as a description of the page rather than a place to make extra claims.