XML

XML (Extensible Markup Language) is, like HTML, a markup language that defines a set of rules for encoding documents in a format that is readable by both humans and machines. The design goals of XML are to emphasize simplicity, generality, and usability within the Internet.

Although the design focus of XML is primarily on documents, it is often used to represent data structures, such as in Web services. Many APIs (application programming interfaces) have been developed for processing XML data by software developers.

Hundreds, if not thousands, of applications have been developed that build on XML. These include RSS, Atom, SOAP, XHTML, and others. Similarly, XML-based formats have become the default for many office applications, including Microsoft Office (Office Open XML), OpenOffice.org and LibreOffice (OpenDocument), and iWork.

In general, there are two current versions of XML. The first version (XML 1.0) was originally defined in 1998, but has since seen several new revisions and is currently in its fifth edition, which was published in November 2008.

The second version of XML (XML 1.1) was originally published in February 2004, on the same day as XML 1.0 Third Edition. XML 1.1 is not as widely used as XML 1.0 and is recommended more for users who need it for its unique features.

An XML document is a kind of string of characters. The characters come from the Unicode repertoire.

Interestingly, the working group that created XML has never met in the real world. They collaborated on the creation exclusively via email and teleconferencing.