XML Schema and Web Services
It should be noted that there are alternatives to defining XML documents beyond XML Schema (for example Schematron and Relax NG). But XML Schema has gotten a lot of traction as an official W3C standard. Equally important, the proliferation of XML Web services very probably means an XML solution to designing documents has a good chance of taking off.
By allowing separate serversregardless of platformto communicate across the Internet in XML, Web services offer a potentially better way to make distributed computing happen. Two XML-based standards, Web Services Description Language (WSDL) and Simple Object Access Protocol (SOAP), are an integral part of Web services. A WSDL document describes the functions that can be called within a particular Web service, while SOAP packages function calls in XML, sending them to a remote server, which then sends an answer back using more XML. Naturally, each side of the conversation requires building a new document type, and the documents are based on an XML Schema, not on DTDs. Web services tools need to generate new XML data types to manage the conversation between servers, and it's only natural that they speak in XML.
Whether you are working with content-based systems, getting XML documents from different organizations or divisions to coexist peacefully, or building Web services, XML Schema has a big role to play. Though mastering the technology is more complicated than that used with old-fashioned DTDs, XML Schemas can offer a more powerful and extensible way to define documents, and they are invaluable for applications that interact using XML. Since you can expect this to happen more and more as Web services take hold, it's a good idea to get started finding out what XML Schema is all about and how it can help your organization do even more with XML.