What is XML? XML is the eXtensible Markup Language. Like HTML, it's a text-based mark-up language that uses tags (i.e. <tag>) to identify data. Unlike HTML, however, it's not a rigid set at all; it is designed to allow you, using SGML (Standard Generalized Markup Language, the parent language of HTML as well as XML), to create your own tags and make your own sets of data, for any type of document you can imagine.

        Since XML is designed to store and transmit data, rather than mark up data for display, these tags tell you what the data is, rather than how to format it. In other words, where HTML tags format things -- i.e. <i> makes things italicized, an XML system might look something like:

<animal>
   <mammal>
       <class>bovinus</class>
       <species>cow</species>
       <speech>Moo!</speech>
   </mammal>
<animal>

(Internet Explorer 5 sorts xml files that don't come with any sort of other formatting in a tree-like format. To see this example in that way, click here.)

        XML documents can be defined to be viewed through an XSL document, or the eXtensible Stylesheet Language. This is the document that tells the XML viewer how to format the data, and what to display. So, if we take the cow XML and filter it through a sample stylesheet by adding the following command right below tag:

<?xml-stylesheet type="text/xsl" href="cow.xsl">

we get this for output. We'll go through this stylesheet line by line to explain what it means in the "how" section.

        So, now that you have a basic concept of what XML and XSL are, why not find out why you'd want to use XML?