There are a number of differences between DTDs and Schema. The most superficial, but still significant, difference is that DTDs use their own syntax, while XML Schema are well-formed, valid XML documents. Some other differences include Schema' capability to use datatyping, their capability to enumerate elements as well as attributes, and their flexibility when implementing occurrence constraints.
It is, however, possible to convert a DTD into an XML Schema, either by stepping through the document, declaration by declaration, and adding Schema improvements when desirable, or by using an automated conversion mechanism, such as a script or application. Either way, converting a DTD into a Schema creates a good base, which can then be extended to offer more flexibility than previously possible.