Electronic Documents Markup
A brief explanation of what markup languages are and where they come from.

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.

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.