XML (eXtensible Markup Language) is a markup language for defining sets of rules for document encodings in a format that is both machine-readable and human-readable.
XML is defined by the W3C's XML 1.0 Specification, and various other related specifications, all of which are free open standards. XML is a textual data format designed to emphasize simplicity, generality, and usability across the Internet. Deviating from its original design defitions, XML was heavily used in the early web to represent arbitrary data structures; it can and still is used for such, although in most cases JSON provides a superior data structure format.
W3C XML Technology Standard Specification:
W3C XML Activity: