Markup in the context of publishing and computer technology refers to the specified display of a text document, its structure, formatting, and the relationship between its parts. Markup is used to enrich a document to facilitate its use by humans.
The standardization of this process is defined within a Markup Language – a set of rules that govern the markup information so that it is readable and interpretable by computer systems. The most used markup languages are HTML, Rich Text Format, PDF and XML.
Schema markup is a specific type of markup language added to HTML documents to provide additional information about the content of the document to computer systems, primarily used by search engines. It is a markup language or schema that is not used for the display of a text document, rather that the display of information in Search Engine Results Pages (SERPs).