XML

XML (Extensible Markup Language) is a standard markup language for documents containing structured information.
Structured information contains both content (text, graphics, etc.) and some indication of the role that the content plays (e.g. content in a section heading has a different meaning from content in a footnote).

XML provides an advantage over HTML as it allows us to make the meaning of the content explicit by identifying structure in the information. HTML is more about the presentation of information.

XML was created so that richly structured documents could be used over the web. Other alternatives are not viable because of a lack of flexibility or because the documents were not originally designed for the web and therefore are not well suited for web applications.
XML is a meta language used to develop new languages, often called applications of XML, which can be classified in different groups:

focused on textual rendering, like HTML;
multimedia oriented, like SVG and SMIL;.
meant for improving information access and navigation, like Topic maps.
standards for information processing and rendering.

More and more grammars are being developed today. Because of the freedom of design and implementation, there's a huge risk to lose accessibility features. For HTML the WAI (Web Accessibility Initiative) has done an excellent job improving web accessibility, but for each of the XML document grammars, designers should take into account demands on accessibility.

A big issue in making information valuable is making things explicit, providing meaningful data structures which improve information understanding, access and navigation.