XML, which stands for Extensible Markup Language, is a markup language that encodes documents in a format that is both human-readable and machine-readable. It is defined by the W3C's XML 1.0 Specification and several other related specifications.
It's designed to be self-descriptive and to be understandable by both humans and machines. It can be used to mark up data and, because it's a text format, can be manipulated by a text editor. XML is one of the most widely-used formats for sharing structured information today.
XML is used extensively to underpin various Internet standards, including RSS, Atom, SOAP, and XHTML. XML is also used as the base language for tools that generate graphical interfaces from an XML description, such as Apache's XML-based GUI generator, Apache XMLForm.