WordPress at a glance
action is not described

user_register action-hook . WP 1.5.0

Fires immediately after a new user is registered.

Usage

add_action( 'user_register', 'action_function_name_7463' );
function action_function_name_7463( $user_id ){
	// action...
}
$user_id(int)
User ID.

Where the hook is called

wp_insert_user()
wp-includes/user.php 1781
do_action( 'user_register', $user_id );

Where the hook is used (in WP core)

wp-admin/includes/admin-filters.php 97
add_action( 'user_register', array( 'WP_Internal_Pointers', 'dismiss_pointers_for_new_users' ) );