Before you continue you should have a basic understanding of the following :

XML was designed to describe data and to focus on what data is. HTML was designed to display data and to focus on how data looks .

HTML is about displaying information, while XML is about describing information.

XML tags are not predefined. You must "invent" your own tags.

The tags used to mark up HTML documents and the structures of HTML documents are predefined. The author of HTML documents can only use tags that are defined in the HTML standard (like , etc.).

XML allows the author to define his own tags and his own document structure.

The tags in the example above (like <to> and <from>) are not defined in any XML standard. These tags are "invented" by the author of the XML document.