If structured data is the language machines understand, then schema markup is the vocabulary of that language. It determines which terms you can use to describe your content. Without a shared vocabulary, every website would mean something different by the same term.
What is schema markup?
Schema markup is a standardised vocabulary from Schema.org, an initiative of Google, Microsoft and Yahoo among others. It defines types and properties: an organisation, a person, a service, an event, a frequently asked question. You use these types in your structured data to describe your content unambiguously.
How does it relate to JSON-LD?
JSON-LD is the format, the how. Schema markup is the content, the what. You write schema types in JSON-LD format. Together they form the complete solution: the right vocabulary in the right format.
A practical example
An estate agency uses the schema type for a local business, with properties for name, address, service area and services. For the frequently asked questions, the agency uses the FAQPage type. This way the search engine knows exactly which part of the page means what.
Why is this important?
Correctly applied schema markup increases the chance of rich search results and of correct interpretation by AI. Incorrectly applied schema markup can actually do harm. It is precise work. That is why a correct implementation and validation is not a luxury but a requirement.
