Extensible Mark-up Language (XML) is a data format that promotes universal access and easy sharing of data. It travels easily from server to server, server to client and application to application. It is simple and flexible in a human-readble form. XML is a subset of SGML, optimised for the web environment which implies data-processing oriented rather than publishing oriented. XML is usually dynamically generated, often created by computer programs and processed by other computer programs, then destroyed.
An XML document is made up from elements, attributes, and one root node. Structural tags reduce complexity of a document. XML is one representation fo electronic data interchange (edi). XML documents have two levels of compliance in that they must be valid (must have a DTD) and well-formed.
Ref: Goldfarb, Charles. F. XML Handbook. 1998