Here’s the brief description about how XML is free and extensible.

• XML is free and extensible

• XML tags are not predefined. We must “invent” our own tags.

• The tags used to mark up HTML documents and the structure of HTML documents are predefined. The author of HTML documents can only use tags that are defined in the HTML standard

XML is not a complement to HTML

• It is important to understand that XML is not a replacement for HTML. In future Web development it is most likely that XML will be used to describe the data, while HTML will be used to format and display the same data.
• XML is a cross-platform, software and hardware independent tool for transmitting information.
• It is important to understand that XML was designed to store, carry,and exchange data. XML was not designed to display data.