title: difference between Mage::app and Mage:: in Magento
body: <p>Can any body say what is the difference between 
  Mage:app and Mage::</p>
  
  <p>eg:</p>
  
  <pre><code>Mage::getModel('catalog/product');
  
  Mage::app-&gt;getLayout-&gt;createBlock('catalog/product_view');
  </code></pre>
  
  <p>if i try like this</p>
  
  <pre><code>Mage::app-&gt;getModel('catalog/product');
  </code></pre>
  
  <p>it throws a fatal error</p>
  
  <p>What is the basic difference?</p>