has two parts:
1) Structures: One part specifies structure relationships.
(This is the largest and most complex part.)
2)Datatypes: The other part specifies mechanisms for validating
the content of XML elements by specifying a (potentially very sophisticated)
datatype for each element.
Advantages over DTD
XML Schema for Structures lets you specify any kind of relationship
you can conceive of.
Disadvantage over DTD
a lot of work to implement, and it takes a bit of learning to use.