Introduction

Extensible Markup Language (XML) provides a significant advance in how data is described and exchanged by applications using a simple, flexible standards-based format. Hypertext markup language (HTML) enables universal methods for viewing data; XML provides universal methods for working directly with data.

XML is the universal language for data on the Web. It gives developers the power to deliver structured data from a wide variety of applications to the desktop for local computation and presentation. XML allows the creation of unique data formats for specific applications. It is also an ideal format for server-to-server transfer of structured data.

The document type definition (DTD) defines the valid syntax of a class of XML documents. That is, it lists a number of element names, which elements can appear in combination with which other ones, what attributes are available for each element type, etc. A DTD uses a different syntax from that used by XML documents.

Schemas combine concepts from DTDs, relational databases, and object-oriented design. Schemas can describe the structure of XML documents, databases, direct-label graphs, and other, similar organizations of data. Schemas supply additional semantic information to documents that are not available in DTDs and contain new facilities such as data types, inheritance, and extensibility. In addition, schemas use the same syntax as XML documents, rather than the unique syntax used by DTDs. Schema components are reusable through the facility of XML namespaces.