Drupal 8: delete all nodes of the same type

One should use entity queries instead of acting directly on the database:

  $result = \Drupal::entityQuery('node')
      ->condition('type', 'my_content_type_name')
      ->execute();
  entity_delete_multiple('node', $result);

Setting up ranges like in the other answer shouldn't be too difficult.

See EntityFieldQuery has been rewritten for more information.

you can use the Devel module

1. go to Admin->configuration->development->Generate content
   ( admin/config/development/generate/content )
2. select the content type you wish to delete its nodes.
3. check "delete all content in these content types.." (important)
4. put "0" in "how many nodes would you like to generate" (important)

see attached image for instructions.

attached image

The entity_delete_multiple is deprecated as of Drupal 8.0.x, will be removed before Drupal 9.0.0. Use the entity storage's delete() method to delete multiple entities:

// query all entities you want for example taxonomy term from tags vocabulary
$query = \Drupal::entityQuery('taxonomy_term');
$query->condition('vid', 'tags');
$tids = $query->execute();

$storage_handler = \Drupal::entityTypeManager()->getStorage($entity_type);
$entities = $storage_handler->loadMultiple($tids);
$storage_handler->delete($entities);