Structured data is a standardised description of the content of a page. A person reads “City bike, price, in stock”; with structured data, the code states that this is a product with a price in euros and a stock status. Google calls it a standardised format for providing information about a page and classifying its content. The vocabulary usually comes from Schema.org.
Which formats are there?
Google supports JSON-LD, Microdata and RDFa, and recommends JSON-LD because it is the easiest to maintain. JSON-LD sits in the HTML as a separate block.
Example
<script type="application/ld+json">
{
"@context": "https://schema.org",
"@type": "Product",
"name": "City bike black",
"offers": { "@type": "Offer", "price": "649.00", "priceCurrency": "EUR" }
}
</script>What does it not do?
Structured data guarantees neither an extra display nor a higher position. It can make a page eligible for rich results, such as a price shown under a search result.
Common mistake
Marking up information the visitor cannot see on the page. Under Google’s guidelines, structured data must be a true representation of the page. A violation can cost a page its eligibility for rich results.
How do you check it?
Test the code with Google’s Rich Results Test. More explanation is in Structured data explained: what it does and what it does not.