Understanding xmlns: what are the links linked to?

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I have read similar question here ,but It still hasn't answered my confusions.

I refer to this tutorial

here's the snipper:

<robot xmlns:sensor="http://playerstage.sourceforge.net/gazebo/xmlschema/#sensor"
   xmlns:controller="http://playerstage.sourceforge.net/gazebo/xmlschema/#controller"
   xmlns:interface="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface"
   xmlns:xacro="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface"
   name="pr2" >

I know that XMLNS is namespace for XML

Quoting gvdhorn's commend on the thread I linked,

"xmlns is short for "XML NameSpace", and is a special XML construct that is expained in the links @VictorLamoine posted. @shawnysh: you can essentially consider them like import statements in the example you posted. But it's really an XML thing, not ROS specific"

Please help to clarify these things:

1. What is the link specified in xmlns linked to? Is this link like a library (implying from gvdhorn's statement)? but why do we need the library?
2. If I am to create my own robot. How is the xmlns somehow involved in the creation?
3. How do i know which link to be linked to?
4. In the tutorial code after specifying the namespace above, could you give examples which lines of code are being appended by the namespace?

Please elaborate

Thank you