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»Next Year's Top 25 Technologies:
Extensible Markup Language (XML)
enables extensible data-exchange formats.«

What is XML?

XML is the 'Extensible Markup Language' (extensible because it is not a fixed format like HTML).

XML is not a single, predefined markup language: it's a metalanguage—a language for describing other languages—which lets you design your own markup. (A predefined markup language like HTML defines a way to describe information in one specific class of documents: XML lets you define your own customized markup languages for different classes of document.) It can do this because it's written in SGML, the international standard metalanguage for markup.