XML

Extensible Markup Language (XML) is a flexible, text based, format derived from SGML. XML was originally designed for satisfying electronic publishing work. XML is a standard for data exchange on the Web and Internet.XML and SGML can be used for describing the structure of metadata.XML documents are made up of storage units called entities, which contain either parsed or unparsed data. Parsed data consists of character data or markup. Markup encodes a description of the document’s storage layout and logical structure. XML provides a mechanism to impose constraints on the storage layout and logical structure.

The are published and maintained by the http://www.w3.org/XML/W3C, they are and freely available on their website.