A file that contains text with interspersed descriptions, called "tags."  All XML files are XML documents and vice versa.  The XML document is often organized in a hierarchy with an "open" tag at the beginning of the file, a "close" tag at the end and all the text elements in between.  Each text element has an open and close tag as well, and all tags begin with the less-than (<) character and end with the greater-than (>) character.  See 
XML.
XML Tag Example
This example shows one definition from the XML feed for this encyclopedia.  All the tags within the open and close "definition" tags are in a prescribed hierarchy.  Note that all close tags have a slash (/) after the beginning less-than (<) symbol.