XML stands for EXtensible Markup Language
XML is a markup language much like HTML
XML was designed to describe data
XML tags are not pre-defined in XML. You must define your own tags.
XML is self-describing
XML uses a DTD (Document Type Definition) to formally describe the data.
As stated before, XML is not a replacement for HTML. If you are still having problems with grasping this aspect, please re-read the introduction to this course.
Another major difference between HTML and XML is that the tags used to markup HTML documents and the structure of HTML documents is pre-defined. An excellent feature of XML is that it allows the authors to define their own tags and the document structure.