The Goals of XML
The main goal of XML was to bring the ability to author structured markup languages to the masses, via the World Wide Web. The technology to create your own markup languages already existed in the form of the Standard Generalized Markup Language, or SGML. However, for many applications, the complexity of SGML was too much, and frightened away many potential users. XML was created to provide a subset of what SGML could provide, allowing developers to create their own markup languages (or XML vocabularies) without all the complexity in SGML. For those users who need advanced features, SGML is still around and useful.
One of the best documents explaining XML succinctly is “XML in 10 Points,” published by the W3C and written by Bert Bos. You can read that document in its entirety at the W3C site (http://www.w3.org/XML/1999/XML-in-10-points); however, here are the main points:
XML is for structuring data— XML documents reflect the structure of the data that they contain. For example, if the document were a book, it might contain <section> elements, which would in turn contain <chapter> elements, and so on.
XML looks a bit like HTML— XML uses tags, both start and end tags, which contain the data in the document and “mark up” the document structure.
XML leads HTML to XHTML— The HTML language of the Web is currently being rewritten to comply with XML. The result will be an XHTML that is case sensitive, and requires all tags to be properly nested and closed.
XML is text, but isn't meant to be read— Although XML is verbose, and it is all ASCII text, XML is still designed primarily to be used by automated systems, not necessarily read by humans.
XML is verbose by design— XML is not designed to be compact; tags can be as long as you would like them to be, in order to make sense. Text is also not the most compact form for storing documents; however, this helps keep XML portable and readable by humans.
XML is a family of technologies— The core of XML is the XML 1.0 Recommendation. However, there are a number of other technologies that contribute to the usefulness of XML. These technologies include the Extensible Stylesheet Language, XML Namespaces, and XML Schemas. In addition, there are XML vocabularies such as Scalable Vector Graphics, which are markup languages written using XML.
XML is new, but not that new— Although XML is relatively new, and subject to much hype, it is based on SGML which is a very well-established technology.
XML is modular— Using XML, you can define vocabularies that are designed to be reused. By creating DTDs or XML Schemas, you can create sets of documents that are all based on common vocabularies. Similarly, using XML Namespaces, you can publish and share those vocabularies without conflicts.
XML is the basis for RDF and the Semantic Web— RDF, or the Resource Description Framework, and the Semantic Web are both initiatives of the W3C to help refine the way information is organized on the Web. XML is the basis of these technologies, and will help organize the information on the Web, making it easier for users to find and access the information they need.
XML is license free, platform independent, and well supported— XML is not owned by any corporation, nor is it controlled by a corporation. It is a publication of the W3C, and as such, it can be used freely by anyone. And although some may have issues with the W3C process, or what ends up in the final Recommendations, the bottom line is that it makes XML a fairly open standard.
The W3C has put a great deal of effort into the development of XML and the XML family of technologies. A quick perusal of the W3C site reveals the level of commitment to XML by the W3C, and the ever-growing XML family. And because XML is text based, and an open recommendation from the W3C, you are not locked into any one vendor's XML solutions; anyone is free to develop solutions using XML, as long as they adhere to the guidelines of the Recommendation.
Note
What is a Recommendation?
Unlike an officially sanctioned standards body, such as the International Standards Organization (ISO), the W3C is not an official standards organization. The W3C simply publishes “Recommendations,” which are not binding in any way. Simply put, they are a set of guidelines, published and copyrighted by the W3C.
The power of these “Recommendations” comes from the fact that people treat them as standards by consensus, and the fact that you can't claim compliance with a Recommendation and not be in compliance without violating the copyrights. Many people are surprised that the W3C has no official status, but many more are shocked to learn that the W3C is not a democracy at all. Although “Recommendations” might be authored by a working group, nothing becomes a final “Recommendation” until signed off by the W3C Director, Tim Berners-Lee.
So, although there are no requirements to follow “standards” released by the W3C as Recommendations, the major players in the Web, such as Netscape and Microsoft, have all agreed to support the W3C.