Schema is a type of microdata that makes it easier for search engines to parse and interpret the information on your web pages more effectively so they can serve relevant results to users based on search queries.

In other words, how is a piece of software going to know what your site is about? Schema is designed to help search engine bots figure that out quickly and correctly.

Schema.org is the centralized home on the web for the Schema project, a collaboration between Google, Bing, Yahoo! and Russian search engine Yandex.

If your site is built on wordpress, there are plugins that will supply the schema markup for you.

It is not exactly necessary, but it does help the search engines determine the rich snippet to display for your page on the search results page. Better rich snippet, better user engagement, the more likely a viewer is to click on your page. Last I heard, Google says schema microdata is not used as a ranking signal, so if you mean necessary for SEO, probably not.

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