XML is the fundamental technology that underpins everything else in Web services. Of paramount importance to the XML suite of technologies is XML Schema, which provides a meta-level description of XML content. XML Schema can, in the simplest sense, be thought of as a means of dictating the format and content of XML documents. However, XML Schema's real power lies in the fact it can be used as a platform independent type description language, where XML documents are then used to transport data in accordance with those type descriptions.
XML technology is already well supported in terms of standard tools. In particular, the XML tools introduced here are widely available across platforms. While the specifics of using most XML tools may vary from platform-to-platform, the models are consistent which means that any experience with such tools is widely applicable.
The sum of these technologies means that XML is not only eminently expressive, but platform independent in the way it is written and processed. As we shall see, this is indeed a rich base on which to build interoperable systems. This is why Web services are based so heavily on XML.