XML documentation.

Extensible Markup Language (XML)
is a markup language that defines a set of rules for encoding documents in a format which is both human-readable and machine-readable.

from Wikipedia

In computing, Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The W3C's XML 1.0 Specification[2] and several other related specifications[3]—all of them free open standards—define XML.[4]

The design goals of XML emphasize simplicity, generality, and usability across the Internet.[5] It is a textual data format with strong support via Unicode for different human languages. Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures[6] such as those used in web services.

Several schema systems exist to aid in the definition of XML-based languages, while programmers have developed many application programming interfaces (APIs) to aid the processing of XML data.

XML Tutorials - techinnovation1
by jesterjunk / 24 videos / 76,309 views / Last updated on Jun 30, 2014

http://www.w3.org/TR/rec-xml