XML (Extensible Markup Language) is a flexible way to create common information formats and share both the format and the data on the World Wide Web, intranets, and elsewhere. For example, computer makers might agree on a standard or common way to describe the information about a computer product (processor speed, memory size, and so forth) and then describe the product information format with XML. Such a standard way of describing data would enable a user to send an intelligent agent (a program) to each computer maker's Web site, gather data, and then make a valid comparison. XML can be used by any individual or group of individuals or companies that wants to share information in a consistent way.

XML Schema

An XML schema is the structural layout of an XML document, expressed in terms of constraints and contents of the document. Constraints are expressed using a combination of the following:

• Grammatical rules governing the order of element
• Data types governing an element and content attribute
• Boolean predicates that the content has to satisfy
• Specialized rules including uniqueness and referential integrity constraints

Document Type Definition

Document type definition (DTD) refers to a set of markup declarations that define a document type for Standard Generalized Markup Language (SGML) languages. DTDs define the structure of a class of documents through element and attribute-list declarations. DTD helps parsers validate documents. It is officially recommended by the World Wide Web Consortium (W3C). DTDs have now been largely superseded by XML Namespace-aware schema languages.