XML, or eXtensible Markup Language, is a meta-language that allows customised mark-up languages to be defined. An XML document has a hierarchical structure, with all the elements of the XML document being represented as members of a pyramidal structure, such that each element is the child element of a parent element, with the exception of the root element. XML looks like HTML, but customised element names can be created.
An XML document is a text file.
The following document shows an XML document that could describe the menu system on a mobile phone.