What is XML?

XML stands for EXtensible Markup Language
XML is a markup language much like HTML
XML was designed to describe data
XML tags are not predefined. You must define your own tags
XML uses a Document Type Definition (DTD) or an XML Schema to describe the data
XML with a DTD or XML Schema is designed to be self-descriptive

The main difference between XML and HTML

XML was designed to carry data.
XML is not a replacement for HTML.
XML and HTML were designed with different goals:

XML was designed to describe data and to focus on what data is.
HTML was designed to display data and to focus on how data looks.

HTML is about displaying information, while XML is about describing information

XML was not designed to DO anything.

Maybe it is a little hard to understand, but XML does not DO anything. XML was created to structure, store and to send information. The following example is a note to Tove from Jani, stored as XML:

	<note>
		<to>Tove</to>
		<from>Jani</fromv
		<heading>Reminder</heading>
		<body>Don't forget me this weekend!</body>
	</note>

The note 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.

XML is free and extensible

XML tags are not predefined. You must "invent" your own tags.

The tags used to mark up HTML documents and the structure of HTML documents is predefined. The author of HTML documents can only use tags that are defined in the HTML standard (like <p>, <h1>, etc.).

XML allows the author to define his own tags and his own document structure.

The tags in the example above (like <to> and <from>) are not defined in any XML standard. These tags are "invented" by the author of the XML document.

The best description of XML is this: XML is a cross-platform, software and hardware independent tool for transmitting information.

http://www.w3schools.com/xml/default.asp