What is XML?

A flexible, extensible data description language
Enables data organization and reuse
Optimized for the World Wide Web
Allows processing of custom tags
Separates content from its presentation
Easily read by humans and parsed by machine
A smaller, more manageable subset of SGML (Standard Generalized Markup Language)

Extensible Markup Language (XML) describes the structure of data, not how data should be displayed. XML's tag syntax is human- and machine-readable, yet device- and database-independent.

XML offers a universal method to work with the data underlying the Web's HTML user interface. Developers can use XML to create new data stores that can be exchanged with any application and viewed in multiple ways.

XML allows processing of custom tags used to define, interpret and transmit data structures between organizations. Since XML is truly extensible, rather than a fixed set of elements like HTML, it may eventually eliminate the need for browser developers to add special HTML tags.

XML 'tags' and 'elements' are not synonymous. An XML element is a whole unit of information with its markup. Tags are simply markers at the start and end of elements. An element may consist of:

a start-tag and an end-tag and the content between them (for example, <CITY>Denver</CITY>)
a start-tag and end-tag immediately next to each other (for example, <configuration></configuration>). This is known as an empty element (an element without content). Empty elements may contain only attributes (properties of the element).
a start tag alone ending with /> instead of > (for example, <configuration/>) - this is the same empty element as above, but represented in an abbreviated form. This shortcut is preferred by most authors. Notice that the forward slash follows the tag name - this is not supported in HTML (compare HTML's <BR> tag). XML requires that an end-tag appear for an empty element as well as for an element with content.

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