XML schema definition languages are derived from the recommendations of the World Wide Web Consortium (W3C). They use XML 1.0 syntax and their role is to explicit describe the structure of XML documents and constrain the data which they may contain. They offer a distinct improvement on the more limited schema features offered by the Document Type Definition (DTD) recommendation which formed part of the original XML specification released in 1998. The most widely used schema language is the one defined by the W3C in 2001: W3C XML Schema. However, there are alternatives, such as RELAX NG and Schematron.

In case you haven't heard of Google Sitemaps (BETA), it is a new program set up by Google to encourage webmasters to create a special XML-format sitemap file for Google to spider through. If you go through the official page, you will find yourself boggled if you are not very good at programming.