Provide a form-specific alteration instead of the global hook_form_alter().
Modules can implement hook_form_FORM_ID_alter() to modify a specific form, rather than implementing hook_form_alter() and checking the form ID, or using long switch statements to alter multiple forms.
Form alter hooks are called in the following order: hook_form_alter(), hook_form_BASE_FORM_ID_alter(), hook_form_FORM_ID_alter(). See hook_form_alter() for more details.
$form: Nested array of form elements that comprise the form.
$form_state: A keyed array containing the current state of the form. The arguments that drupal_get_form() was originally called with are available in the array $form_state['build_info']['args'].
$form_id: String representing the name of the form itself. Typically this is the name of the function that generated the form.
hook_form_BASE_FORM_ID_alter()
Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.
function hook_form_FORM_ID_alter(&$form, &$form_state, $form_id) {
// Modification for the form with the given form ID goes here. For example, if
// FORM_ID is "user_register_form" this code would run only on the user
// registration form.
// Add a checkbox to registration form about agreeing to terms of use.
$form['terms_of_use'] = array(
'#type' => 'checkbox',
'#title' => t("I agree with the website's terms and conditions."),
'#required' => TRUE,
);
}