What exactly is XML NameSpace

[Rohit Wason]

I just went thru http://www.jclark.com/xml/xmlns.htm
Can anybody just explain the idea of XML Namespace? Are we talking about
maintaining a dictionary of <data> tags? What does the term xmlns:HTML="http://www.w3.org/TR/REC-html40
mean? What is contained in the address mentioned above?

Are we talking about some kind of standardization in the field of xml tag
names?

Thanks in advance.
Rohit