With XML schemas, you have more power to define what valid XML documents look like. They have several advantages over DTDs:

• XML schemas use XML syntax. In other words, an XML schema is an XML document. That means you can process a schema just like any other document. For example, you can write an XSLT style sheet that converts an XML schema into a Web form complete with automatically generated JavaScript code that validates the data as you enter it.
• XML schemas support datatypes. While DTDs do support datatypes, it's clear those datatypes were developed from a publishing perspective. XML schemas support all of the original datatypes from DTDs (things like IDs and ID references). They also support integers, floating point numbers, dates, times, strings, URLs, and other datatypes useful for data processing and validation.
• XML schemas are extensible. In addition to the datatypes defined in the XML schema specification, you can also create your own, and you can derive new datatypes based on other datatypes.
• XML schemas have more expressive power. For example, with XML schemas you can define that the value of any <state> attribute can't be longer than 2 characters, or that the value of any <postal-code> element must match the regular expression [0-9]{5}(-[0-9]{4})?. You can't do either of those things with DTDs.