XML XML is the Extensible Markup Language. It is written in Standard Generalized Markup Language (SGML), the international standard metalanguage for text markup systems (ISO 8879). XML can be used to describe customized markups for any type of document because, unlike other SGML-derived markup languages, such as HTML, it is not comprised of a fixed vocabulary of document elements. XML itself is a meta-language used to develop markup languages that describe the content and structure, but not the presentation, of documents.