- 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
The Difference Between XML and HTML
- 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 Does Not DO Anything
Maybe it is a little hard to understand, but XML does not DO anything. XML was created to structure, store, and transport 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 above is quite self descriptive. It has sender and receiver information, it also has a heading and a message body.
But still, this XML document does not DO anything. It is just information wrapped in tags. Someone must write a piece of software to send, receive or display it.
- With XML You Invent Your Own Tags
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.
That is because the XML language has no predefined tags.
The tags used in HTML are predefined. HTML documents can only use tags defined in the HTML standard (like <p>, <h1>, etc.).
- XML allows the author to define his/her own tags and his/her own document structure.
- XML is Not a Replacement for HTML
- XML is a complement to HTML.
It is important to understand that XML is not a replacement for HTML. In most web applications, XML is used to transport data, while HTML is used to format and display the data.
My best description of XML is this:
- XML is a software- and hardware-independent tool for carrying information.
- XML is a W3C Recommendation
- XML became a W3C Recommendation on February 10, 1998.
- XML is Everywhere
- XML is now as important for the Web as HTML was to the foundation of the Web.
- XML is the most common tool for data transmissions between all sorts of applications.
There are 5 predefined entity references in XML:
- < < less than
- > > greater than
- & & ampersand
- ' ' apostrophe
- " " quotation mark
Valid XML Documents
A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a Document Type Definition (DTD):
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE note SYSTEM "Note.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
The DOCTYPE declaration in the example above, is a reference to an external DTD file. The content of the file is shown in the paragraph below.
XML DTD
The purpose of a DTD is to define the structure of an XML document. It defines the structure with a list of legal elements:
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
If you want to study DTD, you will find our DTD tutorial on our homepage.
XML Schema
W3C supports an XML-based alternative to DTD, called XML Schema:
<xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="heading" type="xs:string"/>
<xs:element name="body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
Tip Learn how to create strong passwords. 
