What is XML?

XML (Extensible Markup Language) is a flexible way to create common
information formats and share both the format and the data on the World
Wide Web, intranets, and elsewhere. For example, computer makers might
agree on a standard or common way to describe the information about a computer product
(processor speed, memory size, and so forth) and then describe the product information format
with XML. Such a standard way of describing data would enable a user to send an intelligent
agent (a program) to each computer maker's Web site, gather data, and then make a valid
comparison. XML can be used by any individual or group of individuals or companies that wants
to share information in a consistent way.

Currently a formal recommendation from the World Wide Web Consortium (W3C). XML is
similar to the language of today's Web pages, HTML. Both XML and HTML contain markup
symbols to describe the contents of a page or file. HTML, however, describes the content of a
Web page (mainly text and graphic images) only in terms of how it is to be displayed and
interacted with. For example, a <P> starts a new paragraph. XML describes the content in terms
of what data is being described. For example, a <PHONENUM> could indicate that the data that
followed it was a phone number. This means that an XML file can be processed purely as data by
a program or it can be stored with similar data on another computer or, like an HTML file, that it
can be displayed. For example, depending on how the application in the receiving computer
wanted to handle the phone number, it could be stored, displayed, or dialed.

XML is "extensible" because, unlike HTML, the markup symbols are unlimited and self-defining.
XML is actually a simpler and easier-to-use subset of the Standard Generalized Markup Language
(SGML), the standard for how to create a document structure. It is expected that HTML and
XML will be used together in many Web applications.

Early applications of XML include Microsoft's Channel Definition Format (CDF), which describes
a channel, a portion of a Web site that has been downloaded to your hard disk and is then is
updated periodically as information changes. A specific CDF file contains data that specifies an
initial Web page and how frequently it is updated. Another early application is ChartWare, which
uses XML as a way to describe medical charts so that they can be shared by doctors.
Applications related to banking, e-commerce ordering, personal preference profiles, purchase
orders, litigation documents, part lists, and many others are anticipated.

Related SGML language.

SGML (Standard Generalized Markup Language)

SGML (Standard Generalized Markup Language) is a standard for how to
specify a document markup language or tag set. Such a specification is itself
a document type definition (DTD). SGML is not in itself a document
language, but a description of how to specify one. It is a metalanguage.

SGML is based on the idea that documents have structural and other
semantic elements that can be described without reference to how such
elements should be displayed. The actual display of such a document may vary, depending on the
output medium and style preferences. Some advantages of documents based on SGML are:
 

•  They can be created by thinking in terms of document structure rather than appearance characteristics (which may change over time).
•  They will be more portable because an SGML compiler can interpret any document by reference to its document tag definition (DTD).
•  Documents originally intended for the print medium can easily be re-adapted for other  media, such as the computer display screen.

The language that this Web browser uses, Hypertext Markup Language (HTML), is an example of
an SGML-based language. There is a document type definition for HTML (and reading the
HTML specification is effectively reading an expanded version of the document type definition).

SGML is based somewhat on earlier generalized markup languages developed at IBM, including
General Markup Language (GML) and ISIL.