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 In our XML tutorial you will learn what XML is and the difference between XML and HTML. You will also learn how to start using XML in your applications.
Introduction to XML
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.
What you should already know
Before you continue, you should have some basic understanding of the following:
- WWW, HTML and the basics of building Web pages
- Web scripting languages like JavaScript or VBScript
If you want to study these subjects first, before you start reading about XML, you can find the tutorials you need at W3Schools'''' Home Page.
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 does not DO anything
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</from>
<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 are 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.
XML is a complement to HTML
XML is not a replacement for HTML.
It is important to understand that XML is not a replacement for HTML. In future Web development it is most likely that XML will be used to describe the data, while HTML will be used to format and display the same data.
My best description of XML is this: XML is a cross-platform, software and hardware independent tool for transmitting information.
XML in future Web development
XML is going to be everywhere.
We have been participating in XML development since its creation. It has been amazing to see how quickly the XML standard has been developed and how quickly a 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 most common tool for all data manipulation and data transmission.
XML Joke
Question: When should I use XML?
Answer: When you need a buzzword in your resume.
How can XML be Used? 
It is important to understand that XML was designed to store, carry, and exchange data. XML was not designed to display data.
XML can Separate Data from HTML
With XML, your data is stored outside your HTML.
When HTML is used to display data, the data is stored inside your HTML. With XML, data can be stored in separate XML files. This way you can concentrate on using HTML for data layout and display, and be sure that changes in the underlying data will not require any changes to your HTML.
XML data can also be stored inside HTML pages as "Data Islands". You can still concentrate on using HTML only for formatting and displaying the data.
XML is used to Exchange Data
With XML, data can be exchanged between incompatible systems.
In the real world, computer systems and databases contain data in incompatible formats. One of the most time-consuming challenges for developers has been to exchange data between such systems over the Internet.
Converting the data to XML can greatly reduce this complexity and create data that can be read by many different types of applications.
XML and B2B
With XML, financial information can be exchanged over the Internet.
Expect to see a lot about XML and B2B (Business To Business) in the near future.
XML is going to be the main language for exchanging financial information between businesses over the Internet. A lot of interesting B2B applications are under development.
XML can be used to Share Data
With XML, plain text files can be used to share data.
Since XML data is stored in plain text format, XML provides a software- and hardware-independent way of sharing data.
This makes it much easier to create data that different applications can work with. It also makes it easier to expand or upgrade a system to new operating systems, servers, applications, and new browsers.
XML can be used to Store Data
With XML, plain text files can be used to store data.
XML can also be used to store data in files or in databases. Applications can be written to store and retrieve information from the store, and generic applications can be used to display the data.
XML can make your Data more Useful
With XML, your data is available to more users.
Since XML is independent of hardware, software and application, you can make your data available to other than only standard HTML browsers.
Other clients and applications can access your XML files as data sources, like they are accessing databases. Your data can be made available to all kinds of "reading machines" (agents), and it is easier to make your data available for blind people, or people with other disabilities.
XML can be used to Create new Languages
XML is the mother of WAP and WML.
The Wireless Markup Language (WML), used to markup Internet applications for handheld devices like mobile phones, is written in XML.
You can read more about WML in our WML tutorial.
If Developers have Sense
If they DO have sense, all future applications will exchange their data in XML.
The future might give us word processors, spreadsheet applications and databases that can read each other''''s data in a pure text format, without any conversion utilities in between.
We can only pray that Microsoft and all the other software vendors will agree.
XML Syntax
The syntax rules of XML are very simple and very strict. The rules are very easy to learn, and very easy to use.
Because of this, creating software that can read and manipulate XML is very easy to do.
An example XML document
XML documents use a self-describing and simple syntax.
<?xml version="1.0" encoding="ISO-8859-1"?>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don''''t forget me this weekend!</body>
</note>
The first line in the document - the XML declaration - defines the XML version and the character encoding used in the document. In this case the document conforms to the 1.0 specification of XML and uses the ISO-8859-1 (Latin-1/West European) character set.
The next line describes the root element of the document (like it was saying: "this document is a note"):
<note>
The next 4 lines describe 4 child elements of the root (to, from, heading, and body):
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don''''t forget me this weekend!</body>
And finally the last line defines the end of the root element:
</note>
Can you detect from this example that the XML document contains a Note to Tove from Jani? Don''''t you agree that XML is pretty self-descriptive?
All XML elements must have a closing tag
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