Often, you'll want to add a column to a collection used in a grid, and to do so you'd use _prepareCollection():

protected function _prepareCollection()
{
    $collection = Mage::getResourceModel($this->_getCollectionClass());
  ...

Declare an admin layout XML file for your module in your config.xml:

<config>
    <adminhtml>
        <layout>
            <updates>
                <your_module>
                    <file>your-module.xml</file>
    ...

Frontend With Route:

<config>
    <frontend>
        <routers>
            <namespace_module>
                <use>standard</use>
                <args>
                    <module>Namespace_Module</module>
       ...

Frontend:

<config>
    <frontend>
        <layout>
            <updates>
                <namespace_module>
                    <file>namespace-module.xml</file>
                </namespace_module>
            </up...

/**
 * Yourcompany.com
 *
 * PHP Version 5
 *
 * @category  Namespace
 * @package   Namespace_Module
 * @author    Your Name <your.name@yourcompany.com>
 * @copyright 2012 yourcompany.com
 * @license   http:...

Frontend:

<config>
    <frontend>
        <events>
            <the_name_of_the_event_to_observe>
                <observers>
                    <namespace_module>
                        <class>namespace_module/obser...

Put this in your local.xml:

<config>
    <stores>
        <admin>
            <design>
                 <theme>
                     <default>yourtheme</default>
                 </theme>
            </design>

...

Adding A Tab To The Product Screen

1. Declare an admin layout XML file for your module in your config.xml: