The most important difference between HTML and XML is in HTML you can able to create custom tags while in XML we can.
The XML is transfered over network very easily and it can pass the firewalls also so the data transfer becomes very easy.
With the help of XML it is easy to implement latest technologies like Ajax.
So XML plays vital role in data transfer.
while HTML is only used to display data in browser. It is most flexible format to send over the net.