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XML is the Extensible Markup Language. It improves the functionality of the Web by letting you identify your information in a more accurate, flexible, and adaptable way.

It is extensible because it is not a fixed format like HTML (which is a single, predefined markup language). Instead, XML is actually a metalanguage—a language for describing other languages—which lets you design your own markup languages for limitless different types of documents. XML can do this because it's written in SGML, the international standard metalanguage for text document markup (ISO 8879).

In brief -
  • XML stands for EXtensible Markup Language
  • XML is a markup language much like HTML
  • XML was designed to carry data, not to display data
  • XML tags are not predefined. You must define your own tags
  • XML is designed to be self-descriptive
  • XML is a W3C Recommendation
XML is not a replacement for HTML.

XML and HTML were designed with different goals:

  • XML was designed to transport and store data, with focus on what data is.
  • HTML was designed to display data, with focus on how data looks.

HTML is about displaying information, while XML is about carrying information.
XML is now as important for the Web as HTML was to the foundation of the Web.
  • It is the most common tool for data transmissions between all sorts of applications, and is becoming more and more popular in the area of storing and describing information.
  • In most web applications, XML is used to transport data, while HTML is used to format and display the data.
  • XML allows the author to define his own tags and his own document structure.

XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.
An international standard for markup languages. The basis for HTML and XML.
XML comments are written the same way as HTML comments. i.e.

< !--this is a comment-- >.

The XML processor is not required to pass this information on to the user agent, i.e. the piece of software that is converting the document into some thing useful, but XML also uses CDATA sections which is used to escape blocks of text containing mark up.
XML is used in many aspects of web development, often to simplify data storage and sharing.
  • XML Separates Data from HTML
  • XML Simplifies Data Sharing
  • XML Simplifies Data Transport
  • XML Simplifies Platform Changes
  • XML Makes Your Data More Available
  • XML is Used to Create New Internet Languages
The syntax rules of XML are very simple and logical. The rules are easy to learn, and easy to use.
  • All XML elements must have a closing tag
  • XML tags are case sensitive
  • XML elements must be properly nested
  • XML document must have a root element
  • XML attribute values must be quoted
  • White-space is preserved in XML
  • XML Stores new line as LF
XML elements must follow these naming rules:
  • Names can contain letters, numbers, and other characters
  • Names cannot start with a number or punctuation character
  • Names cannot start with the letters xml (or XML, or Xml, etc)
  • Names cannot contain spaces
Any name can be used, no words are reserved.
A "Well Formed" XML document has correct XML syntax.
For the syntax rule see - What are XML naming rules? in the above question.
Schemas can be included inside of XML file is called Inline Schemas. This is useful when it is inconvenient to physically separate the schema and the XML document. A schema is an XML document that defines the structure, constraints, data types, and relationships of the elements that constitute the data contained inside the XML document or in another XML document. Schema can be an external file which uses the XSD or XDR extension called external schema. Inline schema can take place even when validation is turned off.
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