XML is the Extensible Markup Language. It betters the functionality of the Web by letting you recognize your information in a more accurate, flexible, and adaptable way.
It is extensible due to it is not a fixed format like HTML (which is a one, predefined markup language). Instead, XML is really a Meta language a language for explaining other languages which lets you design your own mark-up languages for limitless dissimilar types of documents. XML can do this because it's written in SGML, the international standard Meta language for text document markup (ISO 8879).