XML The Extensible Markup Language (XML ), which is comparable to SGML and modeled on it, describes how to describe a collection of data. A standard way of describing data It is sometimes referred to as metadata. A specific XML definition defines a set of tags

Both XML and HTML contain markup symbols to describe the contents of a page or file. HTML, however, describes the content of a Web page (mainly text and graphic images) only in terms of how it is to be displayed and interacted with. For example, the letter "p" placed within markup tags starts a new paragraph.

XML describes the content in terms of what data is being described. For example, the word "phonenum" placed within markup tags could indicate that the data that followed was a phone number. An XML tutorial can be found at ibm.com/developerworks/education/xmlintro/xmlintro.html

By applying a different stylesheet to the same document, an XML document can be rendered in different formats. The key is that with XML the information is in the document, while the rendering instructions are elsewhere. In other words, content and presentation are separate. XML is the Web's language for data interchange and HTML is the Web's language for rendering

SGML, DTD, HTML The Standard Generalized Markup Language (SGML ) defines rules for how a document can be described in terms of its logical structure (headings, paragraphs or idea units, and so forth). SGML is often referred to as a metalanguage because it provides a "language for how to describe a language." English grammar and dictionary together could be said to be the metalanguage for English

DTD A specific use of SGML is called a document type definition (DTD). A document type definition spells out exactly what the allowable language is. A DTD is thus a metalanguage for a certain type of document. Hypertext Markup Language (HTML ) is an example of a document type definition.