When talking about structured data, in most cases it means a data format that makes the content of a web page readable for crawlers. They have a standardized format that you use to specify information about a page and classify the page contents.
Google uses such data to more easily capture and classify the content of a page. In addition, data structuring is used to represent particular search result functions and improvements. These are the so-called rich snippets or rich results and also the Google Knowledge Graph.
The schema.org system is used for this purpose. Schema.org is a joint initiative of Google, Microsoft, Yahoo and Yandex with the aim of creating, maintaining and promoting schemas for structured data on the Internet, on websites, in e-mails and beyond.
The Schema.org vocabulary can be used with many different encodings, including RDFa, Microdata, and JSON-LD. These vocabularies include entities, relationships between entities, and actions, and can easily be expanded through a well-documented model.
Over 10 million websites use Schema.org to tag their web pages and email messages. Many applications from Google, Microsoft, Pinterest, Yandex, and others use this vocabulary to provide richer user experiences.