How to create node reference fields in Drupal 7?
The project that maintains the Drupal 7 versions of the "node reference" and "user reference" field types is References.

Part of the code of CCK has been ported in core code, but some code is still living in third-party modules. CCK itself has a version for Drupal 7, which contains the code to update from Drupal 6 to Drupal 7 (among other things).