XML & XHTML...

XML

XML, or Extensible Markup Language, is a flexible way to create common information formats and share both the format and the data on the World Wide Web, intranets, and elsewhere. For example, computer makers might agree on a standard or common way to describe the information about a computer product (processor speed, memory size, and so forth) and then describe the product information format with XML.

Such a standard way of describing data would enable a user to send an intelligent agent (a program) to each computer maker's Web site, gather data, and then make a valid comparison. XML can be used by any individual or group of individuals or companies that wants to share information in a consistent way.

XHTML

The World Wide Web Consortium (W3C) describes XHTML (Extensible Hypertext Markup Language) as "a reformulation of HTML 4.0 as an application of the Extensible Markup Language (XML)."

XML is a structured set of rules for how one might define any kind of data to be shared on the Web. It's called an "extensible" markup¹ language because anyone can invent a particular set of markup for a particular purpose and as long as everyone uses it (the writer and an application program at the receiver's end), it can be adapted and used for many purposes - including, as it happens, describing the appearance of a Web page.

That being the case, it seemed desirable to reframe HTML in terms of XML. The result is XHTML, a particular application of XML for "expressing" Web pages. XHTML is, in fact, the follow-on version of HTML 4. You could think of it as HTML 5, except that it is called XHTML 1.0.

When to use XML

Most web sites won't require the use of XML, however, if you are trying to web-enable an existing application, you may wish to use XML. If you don't want to migrate your old data from an obsolete database, use XML to "wrap" the data in a format which can be interpreted by other more up-to-date web applications.

I can advise on the best method of data encapsulation for web enabling your obsolete applications.

¹ "markup" - a way of representing functionality with tags. ie a carriage return is represented with the "<br>" tag.