HTML is about how the data is to be viewed, displayed, or shown. Thus the "<b> snf </b>" tag in HTML the browser shows the data in between the tags to be bold. It does not understand what the data, in between the tags is all about. It just knows "SHOW THIS DATA BOLD"
XML on the other hand is describes the data. For example the tags <address> </address> one may have a slight hint that the data is an address. Thus xml describes the data.
HTML is also limited to the number of tags it supports. while XML is not.
for further info i suggest you check out
http://java.sun.com/xml/tutorial_intro.html This is the tutorial page of sun for XML
http://java.sun.com/xml/ This is the XML page of sun
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Regds.
Rahul P. Mahindrakar