Extensible Markup Language (XML) is a simple, very flexible text format derived from SGML (ISO 8879). Originally designed to meet the challenges of large-scale electronic publishing, XML is also playing an increasingly important role in the exchange of a wide variety of data on the Web and elsewhere.

The Extensible Markup Language (XML) is a subset of SGML that is completely described in this document. Its goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.

Extensible Markup Language, abbreviated XML, describes a class of data objects called XML documents and partially describes the behavior of computer programs which process them. XML is an application profile or restricted form of SGML, the Standard Generalized Markup Language [ISO 8879]. By construction, XML documents are conforming SGML documents.

XML documents are made up of storage units called entities, which contain either parsed or unparsed data. Parsed data is made up of characters, some of which form character data, and some of which form 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.

[Definition: A software module called an XML processor is used to read XML documents and provide access to their content and structure.] [Definition: It is assumed that an XML processor is doing its work on behalf of another module, called the application.] This specification describes the required behavior of an XML processor in terms of how it must read XML data and the information it must provide to the application.

XML was developed by an XML Working Group (originally known as the SGML Editorial Review Board) formed under the auspices of the World Wide Web Consortium (W3C) in 1996. It was chaired by Jon Bosak of Sun Microsystems with the active participation of an XML Special Interest Group (previously known as the SGML Working Group) also organized by the W3C. The membership of the XML Working Group is given in an appendix. Dan Connolly served as the WG's contact with the W3C. The design goals for XML are:

XML shall be straightforwardly usable over the Internet.
XML shall support a wide variety of applications.
XML shall be compatible with SGML.
It shall be easy to write programs which process XML documents.
The number of optional features in XML is to be kept to the absolute minimum, ideally zero.
XML documents should be human-legible and reasonably clear.
The XML design should be prepared quickly.
The design of XML shall be formal and concise.
XML documents shall be easy to create.
Terseness in XML markup is of minimal importance.

If you are new to XML, perhaps the most confusing aspect is it's similarity to HTML, which makes XML seem familiar at first, but also tends to obscure the view for the finer details of what makes XML tick.

The XML Specification
The W3C specification defines XML as a subset of SGML, so to properly understand XML, it is useful to take a closer look at SGML first. SGML stands for Standard Generalized Markup Language, and was developed for large scale applications, aircraft maintenance or power plant documentation, and intended to be maintained over the long term.

The reason why XML seems to be so similar to HTML lies in the fact that HTML is defined as a subset of SGML. XML is actually a lot more similar to SGML than to HTML, because HTML is only one specific subset of SGML used to describe web pages.

As XML was created to simplify SGML, it is no wonder that the W3C has now decided to redefine HTML 4.0 as an XML application, thereby creating XHTML 1.0. But this shall be of no concern for us at the moment, because we are still faced with the fundamental question "What is XML?".

To answer this, let us define what XML is not:

It is not a programming language.
It is not the next generation of HTML.
It is not a database.
It is not specific to any horizontal or vertical market.
It is not the solution to all your problems, but it can be a very powerful tool in building such a solution.

XML is a clearly defined way to structure, describe, and interchange data. Data in this context really means every conceivable kind of data. You can use XML for such diverse things as describing mathematical formulas, chemical compounds, astronomical information, financial derivatives, architectural blueprints, annotating Shakespearean plays, collecting Buddhist wisdoms, or voice-processing in telephone systems.


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