Extensible Markup Language

Extensible markup language (XML) has rapidly become the "lingua franca" for content representation and system integration. It is likely to become a standard on which much else will be built.

XML is a tagged markup language. Unlike HTML, which is also a tagged language, XML does not have a fixed set of tags. The XML tags can be defined by users, or more often by consortia or internal standards groups. Unlike SGML, from which it descended, XML is much simpler to parse and it is much easier to create tools to manipulate it.

Soon after XML was proposed, the popular press reported that the difference between HTML and XML was that the former was a presentation markup language whereas the latter was a "semantic" markup language. Although there is a grain of truth in that, it has caused a great deal of confusion. XML's tag structure allows you to define your own semantics for a message or a document, but there is nothing about the spec or the language that helps you ascribe any semantic meaning to the tags.

One of the interesting side effects of the ubiquity of XML is that two domains that had previously been distinct, the domain of messages (or transactions) and the domain of documents, can now be expressed in the same syntax. Previously messages or transactions were described in "fixed field" formatted records, electronic data interchange (EDI) between companies, and various proprietary schemes within companies. Documents were described in markup languages such as SGML, if at all. XML wakes us up to the possibility that messages and documents are not really fundamentally different, and a message can be thought of as a small, special-purpose document.

The structure and syntax of the allowable tag combinations in an XML document are defined in its schema. DTD, a carryover from SGML, was the most popular schema language initially. Recently, schemas expressed in XML have become more popular, the most common being XSD.

The presence and popularity of XML have created a torrent of tools and standards that are expressed in XML. Virtually every conceivable industry group has formed consortia and used them to promote an XML-based definition of the standard terms and transactions in its industry. There are hundreds, perhaps thousands, of these industry-led standards in existence already. Additionally, many tool and software package vendors now allow their tools to be parametrically applied via XML and their packages interfaced via XML. Perhaps the most significant development in this area is Web Services.