Structured data is information on a web page written in a fixed, labelled format so software can read it without guessing. A person sees "Open 9 to 5, Monday to Friday" and understands it instantly. A search engine reading the same sentence has to infer meaning, but structured data states it outright: this is a business, these are its opening hours, this is its phone number.
Structured data in SEO: what search engines do with it
In SEO, structured data is the extra layer you add so Google, Bing and other crawlers can classify a page with confidence. It does not change what visitors see. It sits in the page code, usually inside a script tag of type application/ld+json, and describes entities and their properties: a Product with a name, price and availability, or an Event with a start date and location.
Google uses it to understand pages and to decide whether a result qualifies for enhanced displays such as review stars, prices or event dates, which are covered under rich snippets. Google states plainly that adding it does not guarantee those displays appear.
Formats and vocabulary
Two separate things are involved. The vocabulary is the list of allowed types and properties, and on the web that is almost always Schema.org, a shared project started by Google, Microsoft, Yahoo and Yandex. The format is the syntax used to write it. There are three: JSON-LD, Microdata and RDFa. Google recommends JSON-LD because it lives in one block rather than being woven through your HTML attributes. The act of writing that vocabulary into a page is usually called schema markup.
Marking up a bakery's contact page
A bakery's contact page could carry a JSON-LD block declaring type Bakery (a subtype of LocalBusiness), with its street address, telephone, geo coordinates and openingHoursSpecification. Nothing on screen changes. When Google crawls the page, it has an unambiguous record of the address and hours that matches the visible text, which supports local SEO alongside the Google Business Profile.
When a small business should bother
If you sell products online, run events, publish recipes or have a physical location, it is worth adding the relevant types. A five-page brochure site gains a little from Organization or LocalBusiness data and not much more. You can test any page with Google's Rich Results Test or the Schema Markup Validator.
Common structured data mistakes
- Describing things the page does not show. Marking up reviews, prices or FAQs that visitors cannot see breaks Google's guidelines and can lead to a manual action.
- Letting it drift out of date. A price changes on the page but the JSON-LD still states the old one.
- Copying one block to every page. Product data belongs on the product page, not on the homepage and blog posts too.
- Never validating. A missing comma in JSON-LD makes the whole block unreadable.
Related terms
- Schema markup: the practical work of applying the Schema.org vocabulary.
- Rich snippets: the search result upgrades structured data can unlock.
- JSON: the data format JSON-LD is built on.
- Technical SEO: the wider discipline this belongs to.