In this blog post, i will explain how to override magento controllers.
As per magento best practices, we should not change any core files.

So, what we will do is, create a new module called My_Pant and then we will perform our task to override.
Remember: it is a bad programming if core files are modified.
Create config.xml file of your custom module.

Path: "app/code/local/Namespace/Modulename/etc/config.xml" and paste the following contents in that file.
lets suppose namespace is My & module is Pant

<config>

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

 </frontend>

</config>

<My_Pant before="Mage_Checkout">My_Pant</My_Pant> tag will load our custom module's controller files if available.
Now, create a controller file "app/code/local/My/Pant/controllers/CartController.php" and paste the following contents in that file.
 

<?php

require_once 'Mage/Checkout/controllers/CartController.php';
class My_Pant_CartController extends Mage_Checkout_CartController

{

// some code

}