About XML or SGML Formatted Files
Standard Generalized Markup Language (SGML)
SGML is an international standard (ISO 8879) for identifying the structure and content of documents. SGML files are ASCII text files that contain "element identifiers" similar to the tags used in an HTML file. An SGML application also contains a Document Type Definition (DTD) file that defines the document structure and content.
Extensible Markup Language (XML)
Extensible Markup Language (XML) is an SGML application developed with the needs of the Web in mind. It is more flexible than HTML and has fewer restrictions than "pure" SGML. XML has been designed for ease of implementation, and for interoperability with both SGML and HTML. As XML achieves wider implementation in the electronic community, it is expected to replace HTML to produce web documents that are better defined and more visually precise.
More information about XML and SGML can be found on the OASIS Cover Pages, on online resource for markup language technologies.