XML Schema is a newer and more powerful language for specifying what constitutes a valid XML document. DTD lacks support for namespaces and many other features that XML Schema validation offers, which is exactly why many tutorials talk about XML Schema replacing the older DTD.
Then again, tool support for DTD is still going much stronger than that for XML Schema and for many purposes XML Schema is too much added complexity for the added benefit so we'll definitely be seeing DTD documents for years to come.