Structures

Defining Types vs. Elements

Schemas built on a foundation of types Data types are a core feature of XML Schemas, but the typing approach has been extended through the specification. Types can be built from other types, nested, modified... The distinction between declaring types and declarating elements also adds flexibility.
A departure from DTDs While DTDs consist mostly of 'type declarations' there was only a limited number of types and restricted (typically unused) mechanisms for constructing new types.
A familiar model for OOP, though a bit twisted Developers used to working with object-oriented development seem to find XML Schemas much more appealing than DTDs, although more document-centric folk are often bewildered by XML Schemas