What is XML? XML, or eXtensible Markup Language, is a metalanguage. That is, it's a language that can be used to describe other languages and a specification for storing information. Although XML and HTML may seem to have a lot in common, in reality the difference between them is like the difference between night and day. HTML is used to format information, but it isn't very useful when it comes to describing information. For example, you can use HTML to format a table, but you can't use HTML to describe the data elements within the table. The reason for this is that you can't really depict something as abstract as a distributor or a customer with HTML, which is where XML comes into the picture. XML can be, and is, used to define the structure of data rather than its format. This 50-page Fast Start guide begins by showing you the basics of the XML language and then builds on that knowledge to show you how to create an XML document.