The idea is that when you are dealing with XML documents from ten different (external) sources, name collisions can occur. If you use namespaces, you are distinguishing one element from another based on the namespace prefix. myNameSpace:Employee is not the same as yourNameSpace:Employee. When you delcare a namespace you also give it an unique URI ( Universal Resource Identifier ).
Checkout the
Namespace FAQ which is one of my favourite resources.
Good luck,
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Ajith Kallambella M. Sun Certified Programmer for the Java�2 Platform.
IBM Certified Developer - XML and Related Technologies, V1.