Describe the structure of a document.

While HTML is a markup language with a fixed set of tags that allows users to specify the appearance of a document, XML (extensible markup language) allows the user to create new tags to provide a document structure appropriate to the task at hand. A document has three aspects: structure, appearance, and content. In the case of XML, these aspects are separated, with different means employed for defining the structure, the appearance, and the content. Usually, the document type definition (DTD) file describes the structure of a document; the appearance is specified by an extensible style sheet (XSL); and the content is provided in the XML document.