Schema.org is an ontology - a vocabulary for annotating HTML documents with information. For instance, you can put in some info in the web-page markup stating that what you’re displaying is blog post or a list of products.
For instance, a website can present its content and state that this content is an instance of
Blog (say). Schema.org is a central repository describing what
Blog is, what and where it fits in an ontology.
As of 2015, Schema.org is by far the most dominant vocabulary used on the internet .
Schema.org accomplished this feat by aligning incentives correctly. First, annotated webpages would result in a richer search-result presentation - translating to better traffic to the website - a massive economic incentive. Next, the top 4 players in search agreed to support Schema.org - Google was obviously the mothership, bing, Yahoo and Yandex supported it.
Schema.org offers an amazing lesson in setting up the incentives correctly. When you’re building a standard, waiting for people to adopt your rules on the basis of their merit is slightly less efficient than not building the standard at all. A clear economic incentive is a powerful motivator.
The Schema.org team understood that well. And now there is an amazing foundation that is being exploited by a new generation of applications.