The XML FAQ

A markup language is a set of words and symbols for describing the identity of pieces of a document (for example ‘this is a paragraph’, ‘this is a heading’, ‘this is a list’, ‘this is the caption of this figure’, etc). Programs can use this with a stylesheet to create output for screen, print, audio, video, Braille, etc.

Some markup languages (eg those used in wordprocessors) only describe appearances (‘this is italics’, ‘this is bold’), but this method can only be used for display, and is not normally re-usable for anything else.

XML is sometimes referred to as ‘self-describing data’ because the names of the markup elements should represent the type of content they hold.