Extensible Markup Language – a markup language used to display hierarchically structured data, in a text file.

XML data can be opened and edited in an editor. Computers can also read and write XML format. XML consists of tags which are enclosed by pointed brackets: "<" ">". Unlike XML, HTML has only one fixed set of tags, such as, for example <h1>, or <li>. You can define your own custom tags in XML. The standard only defines what a tag must look like, not what it means. Tags can be nested, enabling a hierarchy to be created.

A Document Type Definition (DTD) describes the structure and grammar used in documents. It is an XML system component and is enabled by default.