Appendix C. Understanding XML

Extensible Markup Language (XML) is a markup language for describing and defining structured data independent from application logic. XML is a subset of the more versatile Standard Generalized Markup Language (SGML), which has been the standard, vendor-independent way to maintain repositories of structured documents for more than a decade. Due to its complexity, SGML is not well suited to serving documents over the Internet. XML was developed to include all the important features of SGML, but to leave behind all the drawbacks. Thus, XML retains the power and flexibility of SGML without any of SGML's complexity.

XML is a project of the World Wide Web Consortium (W3C), and the development of the specification is being driven by the XML Working Group. XML is not a single, predefined markup language; rather, it's a meta language in the sense that it enables you to design your own markup. A markup language is a mechanism to identify structures in a document, such as HTML and RTF.

XML, like HTML, is descended from SGML, and both consist of tags. Unlike HTML, however, XML itself doesn't predefine any tags, rather it provides a way to create user-defined tags and the structural relationships between them. The semantics of an XML document either will be defined by the applications that process them or by stylesheets. The elements of HTML are used for presentation and display purposes, whereas elements of XML are used to describe structured data and contents. This appendix covers the fundamentals of XML to help you understand the deployment descriptors of EJBs and applications, which were discussed earlier in the book. For more details, we recommend that you refer to the book, Sams Teach Yourself XML in 24 Hours (ISBN: 0672322137).