If you are familiar with HTML, you have some concept of markup language.If you write a plain text file, it is composed of simple ASCII characters.

Markup languages use different elements to set aside one area of content from other content. The display of these elements (e.g. color, size, and font type) may be determined within the markup file or outside the file using a style sheet. Normally, there is a predetermined set of display characteristics (default) for each element which may be modified locally or using style sheets. Authors are encouraged to separate the determination of display characteristics (style) from the markup file. This makes management of display style much easier but the separation is not required

Markup languages require a Document Type Definition which defines the elements that are allowed in the document. The DTD also defines how elements may be used with relationship to each other. It define how many and which elements may be included inside another element. The DTD is a text file written in a specific format to define the document. The DTD is based on the Standardized Generalized Markup Language (SGML). SGML is the parent language of all markup languages. Although XML may use a DTD, it is not required for those documents that are considered "well formed". A well formed document follows a set of rules for XML and this subject is discussed in more detail later. The DTD also defines another characteristics of the element such as whether or not it requires a beginning or ending tag along with various possible attributes of each element.

XML is a Extensible markup language for documents containing well structured information.

Structured information contains any type of content (words, pictures, etc.) and some indication of what role that content plays (for example, content in a section heading has a different significance from content in a footnote, which means something different than content in a figure caption or content in a database table, etc.). Almost all documents have some structure.

A markup language is a mechanism to identify the document structures. The XML is used to defines a standard way to add markup to documents.

• HTML
• XHTML
• JavaScript / VBScript

In order to appreciate XML, it is very important to understand why it was developed. XML was developed so that richly structured documents could be used over the web. The only alternatives like HTML and SGML, are not practical for this purpose.

HTML,comes bound with a set of syntax / semantics and does not provide arbitrary structure.

SGML provides arbitrary structure, but it is too difficult to implement just for a web browser. Full SGML systems may solve large, complex problems that justify their expense. Viewing a well structured documents sent over the web rarely carries these justification.

XML can not be expected to completely replace SGML. While XML is basically designed to deliver structured content over the web, some of the features it lacks to make this practical, make SGML a more satisfactory solution for the creation and long-time storage of complex documents. In many organizations, filtering SGML to XML will be the standard procedure for web delivery.
 

XML was initially "developed by a World Wide Web Consortium Generic SGML Editorial Review Board formed under the auspices of the W3 Consortium in 1996 and chaired by Jon Bosak of Sun Microsystems, with the active participation of a Generic SGML Working Group also organized by the W3C."

1. To use XML over the Internet, users must be able to view XML documents as quickly and easily as HTML documents. In practice, this will only be possible when XML browsers are as robust and widely available as HTML browsers, but the principle remains.





2. XML support a wide variety of applications. XML should be beneficial to a wide variety of diverse applications: browsing,authoring, content analysis, etc. Although the first focus is on serving structured documents over the web,





3. XML should be compatible with SGML. Most of the people involved in the XML effort come from organizations that have a large, in some cases staggering, amount of material in SGML. XML was designed pragmatically, to be compatible with existing standards while solving the relatively new problem of sending richly structured documents over the web.





4. It should be easy to write programs that process XML documents. The informal way of expressing this goal while the spec was being developed was that it ought to take about two weeks for a competent computer science graduate student to build a program that can process XML documents.





5. Many optional features in XML is to be kept to an absolute minimum, ideally zero. Optional features inevitably raise compatibility problems when users want to share documents and sometimes lead to confusion and frustration.





6. XML documents should be human-legible and reasonably clear. If you don't have an XML browser and you've received a hunk of XML from somewhere, you ought to be able to look at it in your favorite text editor and actually figure out what the content means.





7. The XML design should be prepared quickly. Standards efforts are disreputably slow. XML was needed immediately and was developed as quickly as possible.





8. The design of XML shall be formal and concise. In many ways a notoriously to rule 4, it essentially means that XML must be expressed in Extended Backus-Naur Form (EBNF) and must be amenable to modern compiler tools and techniques.
   There are a number of technical reasons why the SGML grammar cannot be expressed in Extended Backus-Naur Form (EBNF) Writing a proper SGML parser requires handling a variety of rarely used and difficult to parse language features. XML does not.





9. It is easy to create XML documents. Although there will eventually be complicated editors to create and edit XML content, they won't appear immediately. In the interim, it must be possible to create XML documents in other ways: directly in a text editor, with simple shell and Perl scripts, etc.





10. Shortness in XML markup is of minimal importance. Several SGML language features were designed to minimize the amount of typing required to manually key in SGML documents. These features are not supported in XML. From an abstract point of view, these documents are indistinguishable from their more fully specified forms, but supporting these features adds a considerable burden to the SGML parser. In addition, most modern editors offer better facilities to define shortcuts when entering text.

XML is not coming in place of HTML.
XML and HTML were designed with different goals:

XML was designed to illustrate data and to focus on what data is.
HTML was designed to presentation of data and to focus on how data looks.

HTML is about presentation of information, while XML is about describing information.

XML does not DO anything. XML was created to structure, store and to send information.

The following example is a company to Vyom from TCS, stored as XML:

The company has a header and a message body. It also has sender and receiver information. But still, this XML document does not DO anything. It is just pure information wrapped in XML tags. Someone must write a piece of software to send, receive or display it.

There is no XML Standards but how quickly the XML standard has been developed and large number of software vendors have adopted the standard.

We strongly believe that XML will be as important to the future of the Web as HTML has been to the foundation of the Web and that XML will be the important tool for all data manipulation and data transmission.