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What XML Is

• XML stands for eXtensible Markup Language
• XML is a markup language much like HTML (data in between tags example <tag>my data</tag>
• XML is a markup meta-language. A framework for defining other markup languages
• XML was designed to describe data (not how to present it)
• XML tags are NOT predefined. You must define your own tags
• In some cases it is a replacement for EDI

What XML Is Not

• Able to do anything on it's own. It is just plain text with some sort of tag that enclose the data so it can be differentiated from other data.
• Able to define how your data is to be shown. To show data, you need other techniques.
• Able to easily deal with binary data like gifs, jpegs, old MS-Word documents, MS-Excel documents etc.
• A replacement for HTML
• A replacement for relation databases
• A Silver Bullet

XML Basics

• XML files must begin with the XML declaration (prologue)
• Must have one unique root container element.
• All elements must have an opening and matching closing end-tag
• Blank/Empty elements can be coded in a single tag syntax. Example: <MY_TAG_NAME/>
• XML element names and attribute names are case sensitive
• All elements must be properly nested/well formed
• All attribute values must be quoted
• Use the CDATA element for absolute text contents are desired
• HTML comments are supported. Example: <!-- a comment block -->
• XML Namespaces provide a method to avoid element name conflicts. Most XML extension technologies (like SVG, XSLT, XINCLUDE) utilizes a namespace to ensure that their elements do not conflict with contained elements. An example of an "ibis:" namespace is the ibis include element which looks like: <ibis:include href="somedir/somefile.xml" xmlns:ibis="http://www.ibisph.org"/>.
  
  
  IMPORTANT: When a file contains any namespace type element that element or one of its parent elements MUST have a "xmlns:" attribute declaration. In the case of the "ibis:" namespace this would look like this (again an attribute of the current element or a parent element): xmlns:ibis="http://www.ibisph.org". The value of the attribute can be anything unique and must be consistent throughout the file (the URL value is NOT validated/verified so it can be anything just unique and consistent).

XML Resources

• www.w3schools.com is a good all around website that provides tutorials, examples, and reference material for most web related technologies
• Microsoft's XML Developer Center contains the latest news, blogs, great references and examples
• A good XML technologies overview can be found at The XML Revolution Technologies for the future Web