There is nothing special about XML. It is just plain text with the addition of some XML tags enclosed in angle brackets.
Software that can handle plain text can also handle XML. In a simple text editor, the XML tags will be visible and will not be handled specially.
In an XML-aware application however, the XML tags can be handled specially. The tags may or may not be visible, or have a functional meaning, depending on the nature of the application.
ML Elements are extensible and they have relationships.
XML Elements have simple naming rules.