Electronic Documents Markup
   A brief explanation of what markup languages are and where they come from.
   http://www.cs.wpi.edu/~kal/elecdoc/EDmarkup.html

Project Xanadu
   The original hypertext project. Includes a history of the project and gives credit to the early developers of markup languages.
   http://www.xanadu.com/

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.
   http://yaml.org/

whatis.com: markup
   A short definition.
   http://whatis.techtarget.com/definition/0,289893,sid9_gci212527,00.html