http://docs.woothemes.com/document/introduction-to-hooks-actions-and-filters/

actions_hooks.php

<?php

// Action Hook
add_action('action_name', 'your_function_name');

function your_function_name() {
// Your code
}

// Filter Hook
add_filter('filter_name', 'your_function_name');

function your_function_name( $variable ) {
// Your code
return $variable;
}

?>