Depending on the version of Drupal :
drupal 6:
$nodes = db_query('SELECT nid FROM {node} WHERE type="%s"', $type);
drupal 7:
$nodes = node_load_multiple(array(), array('type' => $type));
drupal 8:
$nids = \Drupal::entityQuery('node')
->condition('type', 'NODETYPE')
->execute();
$nodes = \Drupal::entityTypeManager()
->getStorage('node')
->loadMultiple($nids);