magento overwrite core controller

source

<config>

    <frontend> [1]

        <routers>

            <checkout> [2]

                <args>

                    <modules>

                        <yourModule before="Mage_Checkout">Namespace_Module_Overwrite_Checkout</yourModule> [3]

                    </modules>

                </args>

            </checkout>

        </routers>

    </frontend>

</config>

[1] Depending on the controllers area ,”frontend” or “admin”
[2] Router node of the module to be overridden (look it up in the config.xml of that module)
[3] The tag can be anything, it must be unique within the node.
Your_Module_Overwrite_Checkout (is mapped to directory)
Your/Module/controllers/Overwrite/Checkout/