A markup language combines text and extra information about the text. The extra information, for example about the text's structure or presentation, is expressed using markup, which is intermingled with the primary text. The best-known markup language in modern use is HTML (HyperText Markup Language), one of the foundations of the World Wide Web. Historically, markup was (and is) used in the publishing industry in the communication of printed work between authors, editors, and printers.
Classes of markup languages
Markup languages are often divided into three classes: presentational, procedural, and descriptive.
Project Xanadu - The original hypertext project. Includes a history of the project and gives credit to the early developers of markup languages.
whatis.com: markup - A short definition.
Meta Description: [ Markup refers to the sequence of characters or other symbols that you insert at certain places in a text or word processing file to indicate how the file should look when it is printed or displayed or to describe the document s logical structure. The markup indicators are often called tags. F... ]
YAML Ain't Markup Language (YAML) - A straightforward machine parsable data serialization format designed for human readability and interaction with scripting languages such as Perl and Python.