title: What does Namespace URI exactly means?
body: <p>Let us consider this xml data </p>
  
  <pre><code>&lt;root xmlns:h="http://www.w3.org/TR/html4/"
   xmlns:f="http://www.w3schools.com/furniture"&gt;
  
  &lt;h:table&gt;
    &lt;h:tr&gt;
      &lt;h:td&gt;Apples&lt;/h:td&gt;
      &lt;h:td&gt;Bananas&lt;/h:td&gt;
    &lt;/h:tr&gt;
  &lt;/h:table&gt;
  
  &lt;f:table&gt;
    &lt;f:name&gt;African Coffee Table&lt;/f:name&gt;
    &lt;f:width&gt;80&lt;/f:width&gt;
    &lt;f:length&gt;120&lt;/f:length&gt;
  &lt;/f:table&gt;
  
  &lt;/root&gt;
  </code></pre>
  
  <p>Here NURI for h : "<a href="http://www.w3.org/TR/html4/" rel="nofollow">http://www.w3.org/TR/html4/</a>"<br>
  NURI for f : "<a href="http://www.w3schools.com/furniture" rel="nofollow">http://www.w3schools.com/furniture</a>"<br>
  What does the NURI(Namespace URI) exactly means?. Is it necessary?. What it actually contains?</p>