XML tags are not predefined in XML. You must define your own tags.
XML is self describing.
XML uses a DTD (Document Type Definition) to formally describe the data.
The main difference between XML and HTML
XML is not a replacement for HTML.
XML and HTML were designed with different goals:
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, XML is about describing
information.
XML is extensible
The tags used to markup 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 allows the author to define his own tags and his own document structure.
XML is a complement to HTML
It is important to understand that XML is not a replacement for HTML. In the
future development of the Web it is most likely that XML will be used to
structure and describe the Web data, while HTML will be used to format and
display the same data.
XML in future Web development
We have been participating in XML development since its creation. It
has been amazing to see how quickly the XML standard has been developed, and how
quickly a large number of software vendors have adopted the standard.
We strongly believe that XML will be as important to the future of the Web as
HTML has been to the foundation of the Web. XML is the future
for all data transmission and data manipulation over the Web.