Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable.
The World Wide Web Consortium's XML 1.0 Specification of 1998 and several other related specifications—all of them free open standards—define XML.
The design goals of XML emphasize simplicity, generality, and usability across the Internet. It is a textual data format with strong support via Unicode for different human languages.
Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures such as those used in web services.
Wikipedia - XML
W3C XML homepages
XML 1.0 Specification