example config.xml

<?xml version="1.0" encoding="UTF-8"?>
<config>
    <modules>
        <SmashingMagazine_LogProductUpdate>
            <version>0.0.1</version>
        </SmashingMagazine_LogProductUpdate>
    </modules>

    <!-- Configure our module's behavior in the global scope -->
    <global>
        <adminhtml>
            <!-- Defining an event observer -->
            <events>

                <!-- The code of the event we want to observe -->
                <catalog_product_save_after>

                    <!-- Defining an observer for this event -->
                    <observers>

                        <!--
                            Unique identifier within the
                            catalog_product_save_after node.
                            By convention, we write the module's
                            name in lowercase.
                        -->
                        <smashingmagazine_logproductupdate>

                            <!-- The model to be instantiated -->
                            <class>smashingmagazine_logproductupdate/observer</class>

                            <!-- The method of the class to be called -->
                            <method>logUpdate</method>

                            <!-- The type of class to instantiate -->
                            <type>singleton</type>

                        </smashingmagazine_logproductupdate>

                    </observers>

                </catalog_product_save_after>

            </events>
        </adminhtml>
    </global>

</config>