The Extensible Markup Language (XML) is the universal format for structured documents and data on the Web. [1]

Work on XML started in 1996, and has been a W3C Recommendation since February 1998 [2]. However, the technology is not new - XML is based on SGML, an ISO standard from the 80s. XML combines the best parts of SGML with the benefit of experiences from HTML to produce a standard that is no less powerful than SGML, but is much more regular and simple to use.

The XML syntax is very similar to HTML (both use tags and attributes), but enforces well-formedness - a set of rules to guarantee the consistent syntactical representation of data. While HTML specifies what each tag means (and how the enclosed text will appear in a browser), XML just uses tags to mark up pieces of data, leaving the interpretation of the data up to the application that reads it.

XML makes it easy for a computer to generate data, read data, and ensure that the data structure is unambiguous. XML avoids common pitfalls in language design: it is extensible, platform-independent, and it supports internationalization and localization. [3]

XML as a MetaData Language

XML describes the structure and content of a document, making it possible for machines to access this structure and content.

XML documents can contain or point to markup declarations that provide a common grammar for a set of documents. This grammar is the document type definition, or DTD. The inclusion of a DTD is optional, although the defining and including a DTD promotes interoperability - other users can produce syntactically equal XML documents that the same software processes can access.

Extensible Markup Language

XML as a MetaData Language

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