XML stands for Extensible Markup Language, is a simple text based file like HTML for that you don't need any special software installed in your computer. Just NotePad or TextPad can be used for XML writing.
XML file is used to store data in a sequential format like database. It is not used to represent data like HTML.
To represent data of a XML file you have to use CSS or XSL (Extensible Style Sheet Language) file.
And to validate data types used in XML file you have to use DTD or XSD file.
This files have to link to XML file to render style or to check the data types.
A HTML file has predefined tags but an XML file has no predefined tags. You can user your own tag names for your XML file.
The first line of an XML file is:
- <?xml version="1.0" encoding="UTF-8"?>
you can learn more about XML and other from
w3schools.
XML link
XML
CSS link
CSS
XSL link
XSLT
DTD link
DTD
and all other tutorials of related files to XML you can get there.