XML Schema definition language
Syntax and declarations
Simple and complex types
An XML DTD is a specific case of a more general concept called a schema definition. The dictionary defines schema as a “general conception of what is common to all members of a class.” A schema definition takes that “conception” and turns it into something concrete that can be used directly by a computer.
There are many types of schema in use in the computer industry, chiefly for databases. DTDs are different in that the class for which they declare “what is common to all members” is a class of XML documents.
The popularity of XML has brought DTDs to entirely new constituencies. The ...