A W3C-maintained standard for a marginally human read-/editable MarkupLanguage. It is a simplified decendant the extremely comprehensive SGML standard for which hardly a single fully compliant parser was ever written. Many of its features, even implemented ones, are hardly used. On the other hand, it lacks various useful features. XML was designed to address these shortcomings while reducing the language specification to a small set of rules in order to be easily and consistently parsable. It lacks features such as CONCUR but adds others such as NameSpaces (as good an idea in a MarkupLanguage as they are in a ProgrammingLanguage).
XML is specialised using a DTD, an XML Schema, or a RelaxNG? schema to describe the structure of data within a XML document. Each specialisation is actually a new language for marking up a particular type of data. Thus DocBook is a specialisation for marking up the text of books, XHTML is a specialisation for marking up web pages, MathML? is a specialisation for marking up mathematical equations, tables and formulae and XSLT is a specialisation for marking up a programming language (a functional programming language expressed in XML).