A set of labels that are embedded within text to distinguish individual elements or groups of elements for display or identification purposes. The labels are typically known as "tags."
For rendering (displaying and printing), markup languages indicate where font and other layout changes start and stop. For content identification, markup languages turn a text document into the equivalent of a database record in which individual data elements can be located for processing. In a database, elements are placed in a predefined structure. In a document, data elements reside in a freeform structure like text and must be identified with tags that mark their beginning and end.
It All Started with SGML
SGML is the granddaddy markup language that served as the foundation for HTML and XML. HTML is used for rendering the document, and XML is used for identifying the content of the document. See XML vocabulary