XML , markup languages such as standard Generalized Markup Language(SGML) and Hypertext Markup Language(HTML) are also available.
SGML allows documents to describe their grammar by specifying the tag set used in the document and the structural relationship that these tags represent. This makes it possible to define individual formats for documents, handle large and complex documents, and manage large information repositories. However, SGML is complex and difficult for developers to master.
eg :
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
HTML enables you to create documents and Web pages that can be read by all web browsers . It uses a set of tags in conformance with the SGML specification.
XML is a subset of SGML. It is designed to ease data exchange over the Internet. Though HTML and XML are markup languages, they have different purposes. HTML is used for data presentation, whereas XML is used for data description and definition.
In short SGML is the parent of every one. Older HTML utilizes SGML and HTML 4.0 uses XHTML which derived from XML. (more...)