If you haven't heard about it before, you soon will. XML Schema has been talked about for quite some time and is now poised to displace DTDs as the preferred way to specify an XML vocabulary. XML Schema has been developed to plug a few holes with DTDs. For instance:

• The DTD syntax is different from XML and therefore requires mastering another notation.
• DTDs don't support data typing; for example, there is no way to specify that an element must contain an integer.
• DTDs do not support inheritance, which would simplify their reuse.
• DTDs do not support namespaces.
  

The W3C released a candidate recommendation for the XML Schema specification in October 2000, and you can check it out at the site below. We'll also spend some time over the next few days looking at what you can do with XML Schema.
XML Schema