A markup language is a system for annotating a document in a way that is syntactically distinguishable from the text. The idea and terminology evolved from the "marking up" of paper manuscripts, i.e., the revision instructions by editors, traditionally written with a blue pencil on authors' manuscripts.
In digital media this "blue pencil instruction text" was replaced by tags, that is, instructions are expressed directly by tags or "instruction text encapsulated by tags." Examples include typesetting instructions such as those found introff, TeX and LaTeX, or structural markers such as XML tags. Markup instructs the software that displays the text to carry out appropriate actions, but is omitted from the version of the text that users see.
Some markup languages, such as the widely used HTML, have pre-defined presentation semantics—meaning that their specification prescribes how to present the structured data. Others, such as XML, do not.
HyperText Markup Language (HTML), one of the document formats of the World Wide Web, is an instance ofSGML (though, strictly, it does not comply with all the rules of SGML), and follows many of the markup conventions used in the publishing industry in the communication of printed work between authors, editors, and printers.
Getting started with schema.org using Microdata
Most webmasters are familiar with HTML tags on their pages. Usually, HTML tags tell the browser how to display the information included in the tag. For example,
<h1>Avatar</h1> tells the browser to display the text string "Avatar" in a heading 1 format. However, the HTML tag doesn't give any information about what that text string means—"Avatar" could refer to the hugely successful 3D movie, or it could refer to a type of profile picture—and this can make it more difficult for search engines to intelligently display relevant content to a user.
Schema.org provides a collection of shared vocabularies webmasters can use to mark up their pages in ways that can be understood by the major search engines: Google, Microsoft, Yandex and Yahoo!
You use the schema.org vocabulary along with the Microdata, RDFa, or JSON-LD formats to add information to your Web content. This guide will help get you up to speed with Microdata and schema.org so that you can start adding markup to your web pages.
Although this guide focusses on Microdata, most examples on the schema.org site show examples in RDFa and JSON-LD too. The basic ideas (types, properties etc.) introduced here are relevant beyond Microdata - take a look at the examples to see how the details compare.