Schema.org – Structured website metadata for search engines. Helps create Rich Snippets (google.com widgets).
- JSON-LD – Metadata using JSON. Google’s prefered markup. ((https://jsonld.com/google-loves-json-ld-seo/))
- Microdata – Part of the HTML Standard
- RDFa – W3C Recommendation for adding metadata to HTML and other XML based documents
- OpenGraph – Metadata for social media (twitter and facebook, etc..)