XML is short form of Extensible Markup Language. It is based on Standard Generalized Markup language and is simple and very flexible language. Extensible Markup Language is a text based markup language, which plays an important role in data interchange on the web and is widely used in professional web design.

In other words, Extensible Markup Language readily is a streamlined form of SGML, especially suitable for the use on the web. It is easier than SGML and more extensible than Hyper Text Markup Language so its name is the extensible markup language.

A markup language does not worry about how the content it describes is formatted but is instead, concerned with accurately describing its contents. Extensible Markup Language is a markup language that can run on any platform, operating system, or environment and is designed to provide developers with a mechanism to describe their content better so it is widely chosen in professional web design.

Originally it was designed for publishing projects but later it has been developed to make exchange of data on the web much easier and more efficient. It does by allowing developers to write their own document type definitions that describe sets of tags and attributes that can be used to describe specific kinds of content.

Document type definitions are mark up language rule books that define what markup elements can be used to describe a document. If one wants to create own tags, define what markup elements can be used to describe a document. If one wants to create own tags then first we have to define a document type definition.

The individual markup language that XML defines is XML vocabularies or applications and XML defines these through document type definitions. Extensible Markup Language is not sufficiently robust and extensive that it can be used to describe content but also for metadata. Other information can be described by metadata. For example card catalog in a library. Each card is considered as an information resource that provides information usually a book or journal. Similarly metadata is organized way of cataloging of web resources making it easier for natives of the web to conduct efficient searches on the millions of existing web pages.

Extensible Markup Language was developed as document designers and content specialist realized that SGML and HTML are simply too limited in scope to handle the many tasks as not all content can be described as paragraphs, lists, tables and forms in professional web design.

The following points describe why XML is better than SGML and HTML. Firstly XML can be used with existing Web protocols and mechanism and it does not impose any additional requirements. XML has been developed with focus on the web. Only those features of SGML were picked up that was needed for web use. Also HTML is too specific, while XML adopts the generic nature of SGML by adding flexibility, to make it extensive for professional web design.