XML allows applications to communicate with each other.
(Web Applications and Desktop Applications)
XML is the 'Link' between applications.
XML is basically a cross platform 'Language' that allow applications to communicate with each other.
It does not matter if the application is PHP, HTML, Java, Visual Basic, C, ASP, ASP.NET, COBOL etc.
XML is cross platform and language independant.
A good example of XML use is XML Web Services.
XML Web Services are really neat.
Take a look at my site
www.vbasic.net and play with some of the simple XML Web Services I have built.
Now, whats neat is you can use these Services right in your web page or application free of charge...All you have to do is reference my Web Service.
Your application calls the Web Service and my XML Web Service serves your application data in the form of XML.
From there you can do anything you want with it.
I could build a complete application with XML Web Services and just give my customer a 'Thin Client'.
All of the Classes, Properties and Methods (The guts of the program) would actually be on my server and would be accessed through XML Web Services.
So a program that is normally 20 megs is now reduced to less than a meg since the real application is on the server.
You can't compare XML with HTML.
They are used for 2 different things.
Hope this helps.