A DTD is:
The XML Document Type Declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition or DTD.
The DTD can point to an external subset containing markup declarations, or can contain the markup declarations directly in an internal subset, or can even do both.
A Schema is:
XML Schemas express shared vocabularies and allow machines to carry out rules made by people. They provide a means for defining the structure, content and semantics of XML documents.
In summary, schemas are a richer and more powerful of describing information than what is possible with DTDs.