In SEO, structured data usually refers to implementing markup on a webpage to provide additional details about the content of the page.This markup helps search engines to understand the content and enables a website to benefit from enhanced search results such as rich snippets, rich cards, carousels and knowledge boxes.
There are standardised formats (syntaxes) of structured data which are supported by search engines. These are:
There are also two different vocabularies: Schema.org (which can be used with Microdata and JSON-LD) and Microformats.org (which is used with Microformats).
The most commonly used approach to structured data for SEO purposes in Schema.org with JSON-LD.