Schema markup, found at Schema(org), is a form of microdata. Once added to a web page, it creates an enhanced description (commonly known as a rich snippet), which appears in search results.
A schema is a collection of database objects (as far as this hour is concerned—tables) associated with one particular database username. This username is called the schema owner, or the owner of the related group of objects. You may have one or multiple schemas in a database.
Schema markup is code that you put on your website to help the search engines return more informative results for users. Schema markup is especially important in the age of Hummingbird and RankBrain. How a search engine interprets the context of a query will determine the quality of a search result. Schema can provide context to an otherwise ambiguous webpage.
Schema markup is code (semantic vocabulary) that you put on your website to help the search engines return more informative results for users. If you've ever used rich snippets, you'll understand exactly what schema markup is all about. Here's an example of a local business that has markup on its event schedule page.