Definition
Structured data is machine-readable information you add to a web page to tell search engines and AI systems explicitly what is on it: that this is a product with this price and stock, an article by this author on this date, an organisation with this address, or an FAQ with these questions. The vocabulary used is schema.org; the common format is JSON-LD, a block of JSON in a script tag in the head or body.
Without structured data, a machine has to infer from the layout what is what. With structured data it does not have to: you name the entities and their properties literally. Google uses it for rich results (stars, prices, breadcrumbs, events, recipes, videos) and to populate the Knowledge Graph. Language models and answer engines use it to recognise entities on your page unambiguously, even though they do not list it as a separate ranking factor.
The most important types for most businesses are Organization (with logo, contact details and sameAs links to profiles), WebSite, WebPage, BreadcrumbList, Article or BlogPosting (with author and dateModified), Product with Offer and AggregateRating, LocalBusiness for physical locations, FAQPage and HowTo, Service, and DefinedTerm for glossary pages. Nesting is allowed: an Article has an author of type Person, who in turn works for an Organization.
Structured data is not a trick. Google's guidelines require the markup to match the visible content: no marking up reviews that are not there, no FAQ schema on a page without an FAQ. Violations lead to a manual action that removes all rich results from your site. Test every implementation with the Rich Results Test and the Schema Markup Validator, and keep an eye on the Enhancements report in Search Console.
For GEO, structured data is mainly a consistency tool: when your brand name, address, products and authors are marked up identically on every page, an unambiguous entity picture emerges. Traze generates the appropriate JSON-LD (Article, FAQPage, BreadcrumbList, Product or Service) for every published page based on its content, so markup and text never drift apart.