Hello Readers,
This tutorial will guide you about overriding mage core file using config.xml. Here i am gonna give an examples which guide you about "how to override magento blocks, Models & controllers. Always remember, it is not a good habit if core files are modified.
Location of config file: "app/code/local/Namespace/Module/etc/config.xml"

Example: Override Magento's Blocks

 <config>
     <global>
          <blocks>
               <catalog>
                <rewrite>
                 <view>Namespace_Module_Block_View</view>
                </rewrite>
               </catalog>
        </blocks>
     </global>
   </config>

<catalog> and <rewrite> node tells magento that we're going to override 'blocks' of 'Catalog' core module.

Now, create a block file "app/code/local/Namespace/Module/block/View.php"

    class Namespace_Module_Block_View extends Mage_Catalog_Block_Category_View
    {
        public function getProductListHtml()
          {
        return $this>getChildHtml('product_list');
          }
    }

Example: Override Magento Models
  

  <config>
      <global>
     <models>
           <checkout>
            <rewrite>
                 <cart>Namespace_Module_Model_Cart</cart>
            </rewrite>
           </checkout>
     </models>
     </global>
    </config>

Now, create a Model file "app/code/local/Namespace/Module/Model/Cart.php" and paste the following code.
 

   class Namespace_Module_Model_Cart extends Mage_Checkout_Model_Cart
    {
        // override code in new class
    }

<checkout> and <rewrite> node tells magento that we're going to override 'model' of 'Checkout' core module.

Example:  Override Magento's Controllers:
In this example we are overidding "Cart" controller class of the core "checkout" module

    <config>
     <frontend>
        <routers>
                <checkout>
                    <args>
                        <modules>
                            <Namespace_Module before="Mage_Checkout">Namespace_Module</Namespace_Module>
                        </modules>
                    </args>
                </checkout>
            </routers>
     </frontend>
    </config>

<Namespace_Module before="Mage_Checkout">Namespace_Module</Namespace_Module> tag tell Magento that firstly load our custom module's controller files before the mage_checkout core files.
Now, create a controller file "app/code/local/Namespace/Module/controllers/CartController.php" and paste the following contents in that file.

    <?php
    require_once 'Mage/Checkout/controllers/CartController.php';
    class Namespace_Module_CartController extends Mage_Checkout_CartController
    {
     public function indexAction()
    {
         // override code
    }
    }